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    problems such as silent failures in `npm outdated` and `npm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')

src/bin/createUserSession.js

@@ -0,0 +1,51 @@
+'use strict';
+
+/*
+ * A tool for generating a test user session which can be used for debugging configs
+ * that require sessions.
+ */
+
+// As of v14, Node.js does not exit when there is an unhandled Promise rejection. Convert an
+// unhandled rejection into an uncaught exception, which does cause Node.js to exit.
+process.on('unhandledRejection', (err) => { throw err; });
+
+const fs = require('fs');
+const path = require('path');
+const querystring = require('querystring');
+const settings = require('../node/utils/Settings');
+const supertest = require('supertest');
+
+(async () => {
+  const api = supertest(`http://${settings.ip}:${settings.port}`);
+
+  const filePath = path.join(__dirname, '../../APIKEY.txt');
+  const apikey = fs.readFileSync(filePath, {encoding: 'utf-8'});
+
+  let res;
+
+  res = await api.get('/api/');
+  const apiVersion = res.body.currentVersion;
+  if (!apiVersion) throw new Error('No version set in API');
+  const uri = (cmd, args) => `/api/${apiVersion}/${cmd}?${querystring.stringify(args)}`;
+
+  res = await api.post(uri('createGroup', {apikey}));
+  if (res.body.code === 1) throw new Error(`Error creating group: ${res.body}`);
+  const groupID = res.body.data.groupID;
+  console.log('groupID', groupID);
+
+  res = await api.post(uri('createGroupPad', {apikey, groupID}));
+  if (res.body.code === 1) throw new Error(`Error creating group pad: ${res.body}`);
+  console.log('Test Pad ID ====> ', res.body.data.padID);
+
+  res = await api.post(uri('createAuthor', {apikey}));
+  if (res.body.code === 1) throw new Error(`Error creating author: ${res.body}`);
+  const authorID = res.body.data.authorID;
+  console.log('authorID', authorID);
+
+  const validUntil = Math.floor(new Date() / 1000) + 60000;
+  console.log('validUntil', validUntil);
+  res = await api.post(uri('createSession', {apikey, groupID, authorID, validUntil}));
+  if (res.body.code === 1) throw new Error(`Error creating session: ${res.body}`);
+  console.log('Session made: ====> create a cookie named sessionID and set the value to',
+      res.body.data.sessionID);
+})();