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/components/ReplyPlaceholder.js

@@ -0,0 +1,33 @@
+import Component from 'flarum/Component';
+import avatar from 'flarum/helpers/avatar';
+import DiscussionControls from 'flarum/utils/DiscussionControls';
+
+/**
+ * The `ReplyPlaceholder` component displays a placeholder for a reply, which,
+ * when clicked, opens the reply composer.
+ *
+ * ### Props
+ *
+ * - `discussion`
+ */
+export default class ReplyPlaceholder extends Component {
+  view() {
+    function triggerClick(e) {
+      $(this).trigger('click');
+      e.preventDefault();
+    }
+
+    const reply = () => {
+      DiscussionControls.replyAction.call(this.props.discussion, true);
+    };
+
+    return (
+      <article className="post reply-post" onclick={reply} onmousedown={triggerClick}>
+        <header className="post-header">
+          {avatar(app.session.user)}
+          Write a Reply...
+        </header>
+      </article>
+    );
+  }
+}