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    problems such as silent failures in `npm outdated` and `npm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

@@ -0,0 +1,64 @@
+'use strict';
+
+describe('timeslider', function () {
+  // create a new pad before each test run
+  beforeEach(function (cb) {
+    helper.newPad(cb);
+  });
+
+  /**
+   * @todo test authorsList
+   */
+  it("Shows a date/time in the timeslider and make sure it doesn't include NaN", async function () {
+    // make some changes to produce 3 revisions
+    const revs = 3;
+
+    for (let i = 0; i < revs; i++) {
+      await helper.edit('a\n');
+    }
+
+    await helper.gotoTimeslider(revs);
+    await helper.waitForPromise(() => helper.contentWindow().location.hash === `#${revs}`);
+
+    // the datetime of last edit
+    const timerTimeLast = new Date(helper.timesliderTimerTime()).getTime();
+
+    // the day of this revision, e.g. August 12, 2020 (stripped the string "Saved")
+    const dateLast = new Date(helper.revisionDateElem().substr(6)).getTime();
+
+    // the label/revision, ie Version 3
+    const labelLast = helper.revisionLabelElem().text();
+
+    // the datetime should be a date
+    expect(Number.isNaN(timerTimeLast)).to.eql(false);
+
+    // the Date object of the day should not be NaN
+    expect(Number.isNaN(dateLast)).to.eql(false);
+
+    // the label should be Version `Number`
+    expect(labelLast).to.be(`Version ${revs}`);
+
+    // Click somewhere left on the timeslider to go to revision 0
+    helper.sliderClick(1);
+
+    // the datetime of last edit
+    const timerTime = new Date(helper.timesliderTimerTime()).getTime();
+
+    // the day of this revision, e.g. August 12, 2020
+    const date = new Date(helper.revisionDateElem().substr(6)).getTime();
+
+    // the label/revision, e.g. Version 0
+    const label = helper.revisionLabelElem().text();
+
+    // the datetime should be a date
+    expect(Number.isNaN(timerTime)).to.eql(false);
+    // the last revision should be newer or have the same time
+    expect(timerTimeLast).to.not.be.lessThan(timerTime);
+
+    // the Date object of the day should not be NaN
+    expect(Number.isNaN(date)).to.eql(false);
+
+    // the label should be Version 0
+    expect(label).to.be('Version 0');
+  });
+});