Make Post component subclasses build on parent content

Extensions may wish to add attributes/content to all posts, regardless of type, by extending methods on the Post component. Now the subclasses will not overwrite, but rather append to, these additions.

js/forum/src/components/Post.js

@@ -92,10 +92,10 @@ export default class Post extends Component {
   /**
    * Get the post's content.
    *
-   * @return {Object}
+   * @return {Array}
    */
   content() {
-    return '';
+    return [];
   }
 
   /**