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/ReplyComposer.js

@@ -0,0 +1,93 @@
+import ComposerBody from 'flarum/components/ComposerBody';
+import Alert from 'flarum/components/Alert';
+import Button from 'flarum/components/Button';
+import icon from 'flarum/helpers/icon';
+
+/**
+ * The `ReplyComposer` component displays the composer content for replying to a
+ * discussion.
+ *
+ * ### Props
+ *
+ * - All of the props of ComposerBody
+ * - `discussion`
+ */
+export default class ReplyComposer extends ComposerBody {
+  static initProps(props) {
+    super.initProps(props);
+
+    props.placeholder = props.placeholder || 'Write a Reply...';
+    props.submitLabel = props.submitLabel || 'Post Reply';
+    props.confirmExit = props.confirmExit || 'You have not posted your reply. Do you wish to discard it?';
+  }
+
+  headerItems() {
+    const items = super.headerItems();
+    const discussion = this.props.discussion;
+
+    items.add('title', (
+      <h3>
+        {icon('reply')} <a href={app.route.discussion(discussion)} config={m.route}>{discussion.title()}</a>
+      </h3>
+    ));
+
+    return items;
+  }
+
+  /**
+   * Get the data to submit to the server when the reply is saved.
+   *
+   * @return {Object}
+   */
+  data() {
+    return {
+      content: this.content(),
+      relationships: {discussion: this.props.discussion}
+    };
+  }
+
+  onsubmit() {
+    const discussion = this.props.discussion;
+
+    this.loading = true;
+    m.redraw();
+
+    const data = this.data();
+
+    app.store.createRecord('posts').save(data).then(
+      post => {
+        // If we're currently viewing the discussion which this reply was made
+        // in, then we can update the post stream.
+        if (app.viewingDiscussion(discussion)) {
+          app.current.stream.update();
+        } else {
+          // Otherwise, we'll create an alert message to inform the user that
+          // their reply has been posted, containing a button which will
+          // transition to their new post when clicked.
+          let alert;
+          const viewButton = Button.component({
+            children: 'View',
+            onclick: () => {
+              m.route(app.route.post(post));
+              app.alerts.dismiss(alert);
+            }
+          });
+          app.alerts.show(
+            alert = new Alert({
+              type: 'success',
+              message: 'Your reply was posted.',
+              controls: [viewButton]
+            })
+          );
+        }
+
+        app.composer.hide();
+      },
+      errors => {
+        this.loading = false;
+        m.redraw();
+        app.alertErrors(errors);
+      }
+    );
+  }
+}