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/Activity.js

@@ -0,0 +1,57 @@
+import Component from 'flarum/Component';
+import humanTime from 'flarum/helpers/humanTime';
+import avatar from 'flarum/helpers/avatar';
+
+/**
+ * The `Activity` component represents a piece of activity of a user's activity
+ * feed. Subclasses should implement the `description` and `content` methods.
+ *
+ * ### Props
+ *
+ * - `activity`
+ *
+ * @abstract
+ */
+export default class Activity extends Component {
+  view() {
+    const activity = this.props.activity;
+
+    return (
+      <div className="activity">
+        {avatar(this.user(), {className: 'activity-icon'})}
+
+        <div className="activity-info">
+          <strong>{this.description()}</strong>
+          {humanTime(activity.time())}
+        </div>
+
+        {this.content()}
+      </div>
+    );
+  }
+
+  /**
+   * Get the user whose avatar should be displayed.
+   *
+   * @return {User}
+   */
+  user() {
+    return this.props.activity.user();
+  }
+
+  /**
+   * Get the description of the activity.
+   *
+   * @return {VirtualElement}
+   */
+  description() {
+  }
+
+  /**
+   * Get the content to show below the activity description.
+   *
+   * @return {VirtualElement}
+   */
+  content() {
+  }
+}