restructure: move bin/ and tests/ to src/

Also add symlinks from the old `bin/` and `tests/` locations to avoid
breaking scripts and other tools.

Motivations:

  * Scripts and tests no longer have to do dubious things like:

        require('ep_etherpad-lite/node_modules/foo')

    to access packages installed as dependencies in
    `src/package.json`.

  * Plugins can access the backend test helper library in a non-hacky
    way:

        require('ep_etherpad-lite/tests/backend/common')

  * We can delete the top-level `package.json` without breaking our
    ability to lint the files in `bin/` and `tests/`.

    Deleting the top-level `package.json` has downsides: It will cause
    `npm` to print warnings whenever plugins are installed, npm will
    no longer be able to enforce a plugin's peer dependency on
    ep_etherpad-lite, and npm will keep deleting the
    `node_modules/ep_etherpad-lite` symlink that points to `../src`.

    But there are significant upsides to deleting the top-level
    `package.json`: It will drastically speed up plugin installation
    because `npm` doesn't have to recursively walk the dependencies in
    `src/package.json`. Also, deleting the top-level `package.json`
    avoids npm's horrible dependency hoisting behavior (where it moves
    stuff from `src/node_modules/` to the top-level `node_modules/`
    directory). Dependency hoisting causes numerous mysterious
    problems such as silent failures in `npm outdated` and `npm
    update`. Dependency hoisting also breaks plugins that do:

        require('ep_etherpad-lite/node_modules/foo')

tests/frontend/travis/remote_runner.js

@@ -1,183 +0,0 @@
-'use strict';
-
-const wd = require('ep_etherpad-lite/node_modules/wd');
-const async = require('ep_etherpad-lite/node_modules/async');
-
-const config = {
-  host: 'ondemand.saucelabs.com',
-  port: 80,
-  username: process.env.SAUCE_USER,
-  accessKey: process.env.SAUCE_ACCESS_KEY,
-};
-
-let allTestsPassed = true;
-// overwrite the default exit code
-// in case not all worker can be run (due to saucelabs limits),
-// `queue.drain` below will not be called
-// and the script would silently exit with error code 0
-process.exitCode = 2;
-process.on('exit', (code) => {
-  if (code === 2) {
-    console.log('\x1B[31mFAILED\x1B[39m Not all saucelabs runner have been started.');
-  }
-});
-
-const sauceTestWorker = async.queue((testSettings, callback) => {
-  const browser = wd.promiseChainRemote(
-      config.host, config.port, config.username, config.accessKey);
-  const name =
-      `${process.env.GIT_HASH} - ${testSettings.browserName} ` +
-      `${testSettings.version}, ${testSettings.platform}`;
-  testSettings.name = name;
-  testSettings.public = true;
-  testSettings.build = process.env.GIT_HASH;
-  // console.json can be downloaded via saucelabs,
-  // don't know how to print them into output of the tests
-  testSettings.extendedDebugging = true;
-  testSettings.tunnelIdentifier = process.env.TRAVIS_JOB_NUMBER;
-
-  browser.init(testSettings).get('http://localhost:9001/tests/frontend/', () => {
-    const url = `https://saucelabs.com/jobs/${browser.sessionID}`;
-    console.log(`Remote sauce test '${name}' started! ${url}`);
-
-    // tear down the test excecution
-    const stopSauce = (success, timesup) => {
-      clearInterval(getStatusInterval);
-      clearTimeout(timeout);
-
-      browser.quit(() => {
-        if (!success) {
-          allTestsPassed = false;
-        }
-
-        // if stopSauce is called via timeout
-        // (in contrast to via getStatusInterval) than the log of up to the last
-        // five seconds may not be available here. It's an error anyway, so don't care about it.
-        printLog(logIndex);
-
-        if (timesup) {
-          console.log(`[${testSettings.browserName} ${testSettings.platform}` +
-            `${testSettings.version === '' ? '' : (` ${testSettings.version}`)}]` +
-            ' \x1B[31mFAILED\x1B[39m allowed test duration exceeded');
-        }
-        console.log(`Remote sauce test '${name}' finished! ${url}`);
-
-        callback();
-      });
-    };
-
-    /**
-       * timeout if a test hangs or the job exceeds 14.5 minutes
-       * It's necessary because if travis kills the saucelabs session due to inactivity,
-       * we don't get any output
-       * @todo this should be configured in testSettings, see
-       * https://wiki.saucelabs.com/display/DOCS/Test+Configuration+Options#TestConfigurationOptions-Timeouts
-       */
-    const timeout = setTimeout(() => {
-      stopSauce(false, true);
-    }, 870000); // travis timeout is 15 minutes, set this to a slightly lower value
-
-    let knownConsoleText = '';
-    // how many characters of the log have been sent to travis
-    let logIndex = 0;
-    const getStatusInterval = setInterval(() => {
-      browser.eval("$('#console').text()", (err, consoleText) => {
-        if (!consoleText || err) {
-          return;
-        }
-        knownConsoleText = consoleText;
-
-        if (knownConsoleText.indexOf('FINISHED') > 0) {
-          const match = knownConsoleText.match(
-              /FINISHED.*([0-9]+) tests passed, ([0-9]+) tests failed/);
-          // finished without failures
-          if (match[2] && match[2] === '0') {
-            stopSauce(true);
-
-            // finished but some tests did not return or some tests failed
-          } else {
-            stopSauce(false);
-          }
-        } else {
-          // not finished yet
-          printLog(logIndex);
-          logIndex = knownConsoleText.length;
-        }
-      });
-    }, 5000);
-
-    /**
-       * Replaces color codes in the test runners log, appends
-       * browser name, platform etc. to every line and prints them.
-       *
-       * @param {number} index offset from where to start
-       */
-    const printLog = (index) => {
-      let testResult = knownConsoleText.substring(index)
-          .replace(/\[red\]/g, '\x1B[31m').replace(/\[yellow\]/g, '\x1B[33m')
-          .replace(/\[green\]/g, '\x1B[32m').replace(/\[clear\]/g, '\x1B[39m');
-      testResult = testResult.split('\\n').map((line) => `[${testSettings.browserName} ` +
-        `${testSettings.platform}` +
-        `${testSettings.version === '' ? '' : (` ${testSettings.version}`)}]` +
-        `${line}`).join('\n');
-
-      console.log(testResult);
-    };
-  });
-}, 6); // run 6 tests in parrallel
-
-// 1) Firefox on Linux
-sauceTestWorker.push({
-  platform: 'Windows 7',
-  browserName: 'firefox',
-  version: '52.0',
-});
-
-// 2) Chrome on Linux
-sauceTestWorker.push({
-  platform: 'Windows 7',
-  browserName: 'chrome',
-  version: '55.0',
-  args: ['--use-fake-device-for-media-stream'],
-});
-
-/*
-// 3) Safari on OSX 10.15
-sauceTestWorker.push({
-    'platform'       : 'OS X 10.15'
-  , 'browserName'    : 'safari'
-  , 'version'        : '13.1'
-});
-*/
-
-// 4) Safari on OSX 10.14
-sauceTestWorker.push({
-  platform: 'OS X 10.15',
-  browserName: 'safari',
-  version: '13.1',
-});
-// IE 10 doesn't appear to be working anyway
-/*
-// 4) IE 10 on Win 8
-sauceTestWorker.push({
-    'platform'       : 'Windows 8'
-  , 'browserName'    : 'iexplore'
-  , 'version'        : '10.0'
-});
-*/
-// 5) Edge on Win 10
-sauceTestWorker.push({
-  platform: 'Windows 10',
-  browserName: 'microsoftedge',
-  version: '83.0',
-});
-// 6) Firefox on Win 7
-sauceTestWorker.push({
-  platform: 'Windows 7',
-  browserName: 'firefox',
-  version: '78.0',
-});
-
-sauceTestWorker.drain(() => {
-  process.exit(allTestsPassed ? 0 : 1);
-});