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    problems such as silent failures in `npm outdated` and `npm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/frontend/specs/chat_load_messages.js

@@ -0,0 +1,80 @@
+'use strict';
+
+describe('chat-load-messages', function () {
+  let padName;
+
+  it('creates a pad', function (done) {
+    padName = helper.newPad(done);
+    this.timeout(60000);
+  });
+
+  it('adds a lot of messages', function (done) {
+    const chrome$ = helper.padChrome$;
+    const chatButton = chrome$('#chaticon');
+    chatButton.click();
+    const chatInput = chrome$('#chatinput');
+    const chatText = chrome$('#chattext');
+
+    this.timeout(60000);
+
+    const messages = 140;
+    for (let i = 1; i <= messages; i++) {
+      let num = `${i}`;
+      if (num.length === 1) num = `00${num}`;
+      if (num.length === 2) num = `0${num}`;
+      chatInput.sendkeys(`msg${num}`);
+      chatInput.sendkeys('{enter}');
+    }
+    helper.waitFor(() => chatText.children('p').length === messages, 60000).always(() => {
+      expect(chatText.children('p').length).to.be(messages);
+      helper.newPad(done, padName);
+    });
+  });
+
+  it('checks initial message count', function (done) {
+    let chatText;
+    const expectedCount = 101;
+    const chrome$ = helper.padChrome$;
+    helper.waitFor(() => {
+      const chatButton = chrome$('#chaticon');
+      chatButton.click();
+      chatText = chrome$('#chattext');
+      return chatText.children('p').length === expectedCount;
+    }).always(() => {
+      expect(chatText.children('p').length).to.be(expectedCount);
+      done();
+    });
+  });
+
+  it('loads more messages', function (done) {
+    const expectedCount = 122;
+    const chrome$ = helper.padChrome$;
+    const chatButton = chrome$('#chaticon');
+    chatButton.click();
+    const chatText = chrome$('#chattext');
+    const loadMsgBtn = chrome$('#chatloadmessagesbutton');
+
+    loadMsgBtn.click();
+    helper.waitFor(() => chatText.children('p').length === expectedCount).always(() => {
+      expect(chatText.children('p').length).to.be(expectedCount);
+      done();
+    });
+  });
+
+  it('checks for button vanishing', function (done) {
+    const expectedDisplay = 'none';
+    const chrome$ = helper.padChrome$;
+    const chatButton = chrome$('#chaticon');
+    chatButton.click();
+    const loadMsgBtn = chrome$('#chatloadmessagesbutton');
+    const loadMsgBall = chrome$('#chatloadmessagesball');
+
+    loadMsgBtn.click();
+    helper.waitFor(() => loadMsgBtn.css('display') === expectedDisplay &&
+             loadMsgBall.css('display') === expectedDisplay).always(() => {
+      expect(loadMsgBtn.css('display')).to.be(expectedDisplay);
+      expect(loadMsgBall.css('display')).to.be(expectedDisplay);
+      done();
+    });
+  });
+});