Clean up app.current, app.previous in JS (#2156)

- Encapsulate app.current, app.previous in PageState objects
- Reorganize Page classes to use one central base class in common

Co-authored-by: Franz Liedke <franz@develophp.org>

js/src/forum/ForumApplication.js

@@ -160,7 +160,7 @@ export default class ForumApplication extends Application {
    * @return {Boolean}
    */
   viewingDiscussion(discussion) {
-    return this.current instanceof DiscussionPage && this.current.discussion === discussion;
+    return this.current.matches(DiscussionPage, { discussion });
   }
 
   /**