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    problems such as silent failures in `npm outdated` and `npm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/backend/specs/api/chat.js

@@ -1,112 +0,0 @@
-function m(mod) { return `${__dirname}/../../../../src/${mod}`; }
-
-const common = require('../../common');
-const settings = require(m('node/utils/Settings'));
-const supertest = require(m('node_modules/supertest'));
-
-const api = supertest(`http://${settings.ip}:${settings.port}`);
-const apiKey = common.apiKey;
-let apiVersion = 1;
-let authorID = '';
-const padID = makeid();
-const timestamp = Date.now();
-
-describe(__filename, function () {
-  describe('API Versioning', function () {
-    it('errors if can not connect', function (done) {
-      api.get('/api/')
-          .expect((res) => {
-            apiVersion = res.body.currentVersion;
-            if (!res.body.currentVersion) throw new Error('No version set in API');
-            return;
-          })
-          .expect(200, done);
-    });
-  });
-
-  // BEGIN GROUP AND AUTHOR TESTS
-  // ///////////////////////////////////
-  // ///////////////////////////////////
-
-  /* Tests performed
-  -> createPad(padID)
-   -> createAuthor([name]) -- should return an authorID
-    -> appendChatMessage(padID, text, authorID, time)
-     -> getChatHead(padID)
-      -> getChatHistory(padID)
-  */
-
-  describe('createPad', function () {
-    it('creates a new Pad', function (done) {
-      api.get(`${endPoint('createPad')}&padID=${padID}`)
-          .expect((res) => {
-            if (res.body.code !== 0) throw new Error('Unable to create new Pad');
-          })
-          .expect('Content-Type', /json/)
-          .expect(200, done);
-    });
-  });
-
-  describe('createAuthor', function () {
-    it('Creates an author with a name set', function (done) {
-      api.get(endPoint('createAuthor'))
-          .expect((res) => {
-            if (res.body.code !== 0 || !res.body.data.authorID) throw new Error('Unable to create author');
-            authorID = res.body.data.authorID; // we will be this author for the rest of the tests
-          })
-          .expect('Content-Type', /json/)
-          .expect(200, done);
-    });
-  });
-
-  describe('appendChatMessage', function () {
-    it('Adds a chat message to the pad', function (done) {
-      api.get(`${endPoint('appendChatMessage')}&padID=${padID}&text=blalblalbha&authorID=${authorID}&time=${timestamp}`)
-          .expect((res) => {
-            if (res.body.code !== 0) throw new Error('Unable to create chat message');
-          })
-          .expect('Content-Type', /json/)
-          .expect(200, done);
-    });
-  });
-
-
-  describe('getChatHead', function () {
-    it('Gets the head of chat', function (done) {
-      api.get(`${endPoint('getChatHead')}&padID=${padID}`)
-          .expect((res) => {
-            if (res.body.data.chatHead !== 0) throw new Error('Chat Head Length is wrong');
-
-            if (res.body.code !== 0) throw new Error('Unable to get chat head');
-          })
-          .expect('Content-Type', /json/)
-          .expect(200, done);
-    });
-  });
-
-  describe('getChatHistory', function () {
-    it('Gets Chat History of a Pad', function (done) {
-      api.get(`${endPoint('getChatHistory')}&padID=${padID}`)
-          .expect((res) => {
-            if (res.body.data.messages.length !== 1) throw new Error('Chat History Length is wrong');
-            if (res.body.code !== 0) throw new Error('Unable to get chat history');
-          })
-          .expect('Content-Type', /json/)
-          .expect(200, done);
-    });
-  });
-});
-
-var endPoint = function (point) {
-  return `/api/${apiVersion}/${point}?apikey=${apiKey}`;
-};
-
-function makeid() {
-  let text = '';
-  const possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
-
-  for (let i = 0; i < 5; i++) {
-    text += possible.charAt(Math.floor(Math.random() * possible.length));
-  }
-  return text;
-}