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    problems such as silent failures in `npm outdated` and `npm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

@@ -0,0 +1,191 @@
+'use strict';
+
+/**
+ * the contentWindow is either the normal pad or timeslider
+ *
+ * @returns {HTMLElement} contentWindow
+ */
+helper.contentWindow = function () {
+  return $('#iframe-container iframe')[0].contentWindow;
+};
+
+/**
+ * Opens the chat unless it is already open via an
+ * icon on the bottom right of the page
+ *
+ * @returns {Promise}
+ */
+helper.showChat = function () {
+  const chaticon = helper.chatIcon();
+  if (chaticon.hasClass('visible')) {
+    chaticon.click();
+    return helper.waitForPromise(() => !chaticon.hasClass('visible'), 2000);
+  }
+};
+
+/**
+ * Closes the chat window if it is shown and not sticky
+ *
+ * @returns {Promise}
+ */
+helper.hideChat = function () {
+  if (helper.isChatboxShown() && !helper.isChatboxSticky()) {
+    helper.titlecross().click();
+    return helper.waitForPromise(() => !helper.isChatboxShown(), 2000);
+  }
+};
+
+/**
+ * Gets the chat icon from the bottom right of the page
+ *
+ * @returns {HTMLElement} the chat icon
+ */
+helper.chatIcon = function () { return helper.padChrome$('#chaticon'); };
+
+/**
+ * The chat messages from the UI
+ *
+ * @returns {Array.<HTMLElement>}
+ */
+helper.chatTextParagraphs = function () { return helper.padChrome$('#chattext').children('p'); };
+
+/**
+ * Returns true if the chat box is sticky
+ *
+ * @returns {boolean} stickyness of the chat box
+ */
+helper.isChatboxSticky = function () {
+  return helper.padChrome$('#chatbox').hasClass('stickyChat');
+};
+
+/**
+ * Returns true if the chat box is shown
+ *
+ * @returns {boolean} visibility of the chat box
+ */
+helper.isChatboxShown = function () {
+  return helper.padChrome$('#chatbox').hasClass('visible');
+};
+
+/**
+ * Gets the settings menu
+ *
+ * @returns {HTMLElement} the settings menu
+ */
+helper.settingsMenu = function () { return helper.padChrome$('#settings'); };
+
+/**
+ * Gets the settings button
+ *
+ * @returns {HTMLElement} the settings button
+ */
+helper.settingsButton = function () {
+  return helper.padChrome$("button[data-l10n-id='pad.toolbar.settings.title']");
+};
+
+/**
+ * Toggles user list
+ */
+helper.toggleUserList = async function () {
+  const isVisible = helper.userListShown();
+  const button = helper.padChrome$("button[data-l10n-id='pad.toolbar.showusers.title']");
+  button.click();
+  await helper.waitForPromise(() => !isVisible);
+};
+
+/**
+ * Gets the user name input field
+ *
+ * @returns {HTMLElement} user name input field
+ */
+helper.usernameField = function () {
+  return helper.padChrome$("input[data-l10n-id='pad.userlist.entername']");
+};
+
+/**
+ * Is the user list popup shown?
+ *
+ * @returns {boolean}
+ */
+helper.userListShown = function () {
+  return helper.padChrome$('div#users').hasClass('popup-show');
+};
+
+/**
+ * Sets the user name
+ *
+ */
+helper.setUserName = async (name) => {
+  const userElement = helper.usernameField();
+  userElement.click();
+  userElement.val(name);
+  userElement.blur();
+  await helper.waitForPromise(() => !helper.usernameField().hasClass('editactive'));
+};
+
+/**
+ * Gets the titlecross icon
+ *
+ * @returns {HTMLElement} the titlecross icon
+ */
+helper.titlecross = function () { return helper.padChrome$('#titlecross'); };
+
+/**
+ * Returns true if the settings menu is visible
+ *
+ * @returns {boolean} is the settings menu shown?
+ */
+helper.isSettingsShown = function () {
+  return helper.padChrome$('#settings').hasClass('popup-show');
+};
+
+/**
+ * Gets the timer div of a timeslider that has the datetime of the revision
+ *
+ * @returns {HTMLElement} timer
+ */
+helper.timesliderTimer = function () {
+  if (typeof helper.contentWindow().$ === 'function') {
+    return helper.contentWindow().$('#timer');
+  }
+};
+
+/**
+ * Gets the time of the revision on a timeslider
+ *
+ * @returns {HTMLElement} timer
+ */
+helper.timesliderTimerTime = function () {
+  if (helper.timesliderTimer()) {
+    return helper.timesliderTimer().text();
+  }
+};
+
+/**
+ * The ui-slidar-bar element in the timeslider
+ *
+ * @returns {HTMLElement}
+ */
+helper.sliderBar = function () {
+  return helper.contentWindow().$('#ui-slider-bar');
+};
+
+/**
+ * revision_date element
+ * like "Saved October 10, 2020"
+ *
+ * @returns {HTMLElement}
+ */
+helper.revisionDateElem = function () {
+  return helper.contentWindow().$('#revision_date').text();
+};
+
+/**
+ * revision_label element
+ * like "Version 1"
+ *
+ * @returns {HTMLElement}
+ */
+helper.revisionLabelElem = function () {
+  return helper.contentWindow().$('#revision_label');
+};