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    problems such as silent failures in `npm outdated` and `npm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/tests/ratelimit/send_changesets.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const etherpad = require('etherpad-cli-client');
+
+const pad = etherpad.connect(process.argv[2]);
+pad.on('connected', () => {
+  setTimeout(() => {
+    setInterval(() => {
+      pad.append('1');
+    }, process.argv[3]);
+  }, 500); // wait because CLIENT_READY message is included in ratelimit
+
+  setTimeout(() => {
+    process.exit(0);
+  }, 11000);
+});
+// in case of disconnect exit code 1
+pad.on('message', (message) => {
+  if (message.disconnect === 'rateLimited') {
+    process.exit(1);
+  }
+});