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    problems such as silent failures in `npm outdated` and `npm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/timeslider_follow.js

@@ -0,0 +1,100 @@
+'use strict';
+
+describe('timeslider follow', function () {
+  // create a new pad before each test run
+  beforeEach(function (cb) {
+    helper.newPad(cb);
+  });
+
+  // TODO needs test if content is also followed, when user a makes edits
+  // while user b is in the timeslider
+  it("content as it's added to timeslider", async function () {
+    // send 6 revisions
+    const revs = 6;
+    const message = 'a\n\n\n\n\n\n\n\n\n\n';
+    const newLines = message.split('\n').length;
+    for (let i = 0; i < revs; i++) {
+      await helper.edit(message, newLines * i + 1);
+    }
+
+    await helper.gotoTimeslider(0);
+    await helper.waitForPromise(() => helper.contentWindow().location.hash === '#0');
+
+    const originalTop = helper.contentWindow().$('#innerdocbody').offset();
+
+    // set to follow contents as it arrives
+    helper.contentWindow().$('#options-followContents').prop('checked', true);
+    helper.contentWindow().$('#playpause_button_icon').click();
+
+    let newTop;
+    await helper.waitForPromise(() => {
+      newTop = helper.contentWindow().$('#innerdocbody').offset();
+      return newTop.top < originalTop.top;
+    });
+  });
+
+  /**
+   * Tests for bug described in #4389
+   * The goal is to scroll to the first line that contains a change right before
+   * the change is applied.
+   */
+  it('only to lines that exist in the pad view, regression test for #4389', async function () {
+    await helper.clearPad();
+    await helper.edit('Test line\n' +
+      '\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n' +
+      '\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n' +
+      '\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n');
+    await helper.edit('Another test line', 40);
+
+
+    await helper.gotoTimeslider();
+
+    // set to follow contents as it arrives
+    helper.contentWindow().$('#options-followContents').prop('checked', true);
+
+    const oldYPosition = helper.contentWindow().$('#editorcontainerbox')[0].scrollTop;
+    expect(oldYPosition).to.be(0);
+
+    /**
+     * pad content rev 0 [default Pad text]
+     * pad content rev 1 ['']
+     * pad content rev 2 ['Test line','','', ..., '']
+     * pad content rev 3 ['Test line','',..., 'Another test line', ..., '']
+     */
+
+    // line 40 changed
+    helper.contentWindow().$('#leftstep').click();
+    await helper.waitForPromise(() => hasFollowedToLine(40));
+
+    // line 1 is the first line that changed
+    helper.contentWindow().$('#leftstep').click();
+    await helper.waitForPromise(() => hasFollowedToLine(1));
+
+    // line 1 changed
+    helper.contentWindow().$('#leftstep').click();
+    await helper.waitForPromise(() => hasFollowedToLine(1));
+
+    // line 1 changed
+    helper.contentWindow().$('#rightstep').click();
+    await helper.waitForPromise(() => hasFollowedToLine(1));
+
+    // line 1 is the first line that changed
+    helper.contentWindow().$('#rightstep').click();
+    await helper.waitForPromise(() => hasFollowedToLine(1));
+
+    // line 40 changed
+    helper.contentWindow().$('#rightstep').click();
+    helper.waitForPromise(() => hasFollowedToLine(40));
+  });
+});
+
+/**
+ * @param {number} lineNum
+ * @returns {boolean} scrolled to the lineOffset?
+ */
+const hasFollowedToLine = (lineNum) => {
+  const scrollPosition = helper.contentWindow().$('#editorcontainerbox')[0].scrollTop;
+  const lineOffset =
+      helper.contentWindow().$('#innerdocbody').find(`div:nth-child(${lineNum})`)[0].offsetTop;
+  return Math.abs(scrollPosition - lineOffset) < 1;
+};