Massive JavaScript cleanup

- Use JSX for templates
- Docblock/comment everything
- Mostly passes ESLint (still some work to do)
- Lots of renaming, refactoring, etc.

CSS hasn't been updated yet.

js/forum/src/ForumApp.js

@@ -0,0 +1,86 @@
+import History from 'flarum/utils/History';
+import App from 'flarum/App';
+import Search from 'flarum/components/Search';
+import Composer from 'flarum/components/Composer';
+import ReplyComposer from 'flarum/components/ReplyComposer';
+import DiscussionPage from 'flarum/components/DiscussionPage';
+
+export default class ForumApp extends App {
+  constructor(...args) {
+    super(...args);
+
+    /**
+     * The app's history stack, which keeps track of which routes the user visits
+     * so that they can easily navigate back to the previous route.
+     *
+     * @type {History}
+     */
+    this.history = new History();
+
+    /**
+     * An object which controls the state of the page's side pane.
+     *
+     * @type {Pane}
+     */
+    this.pane = null;
+
+    /**
+     * The page's search component instance.
+     *
+     * @type {SearchBox}
+     */
+    this.search = new Search();
+
+    /**
+     * An object which controls the state of the page's drawer.
+     *
+     * @type {Drawer}
+     */
+    this.drawer = null;
+
+    /**
+     * A map of post types to their components.
+     *
+     * @type {Object}
+     */
+    this.postComponents = {};
+
+    /**
+     * A map of activity types to their components.
+     *
+     * @type {Object}
+     */
+    this.activityComponents = {};
+
+    /**
+     * A map of notification types to their components.
+     *
+     * @type {Object}
+     */
+    this.notificationComponents = {};
+  }
+
+  /**
+   * Check whether or not the user is currently composing a reply to a
+   * discussion.
+   *
+   * @param {Discussion} discussion
+   * @return {Boolean}
+   */
+  composingReplyTo(discussion) {
+    return this.composer.component instanceof ReplyComposer &&
+      this.composer.component.props.discussion === discussion &&
+      this.composer.position !== Composer.PositionEnum.HIDDEN;
+  }
+
+  /**
+   * Check whether or not the user is currently viewing a discussion.
+   *
+   * @param {Discussion} discussion
+   * @return {Boolean}
+   */
+  viewingDiscussion(discussion) {
+    return this.current instanceof DiscussionPage &&
+      this.current.discussion === discussion;
+  }
+}