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/states/GlobalSearchState.js

@@ -77,7 +77,7 @@ export default class GlobalSearchState extends SearchState {
    * @return {String}
    */
   getInitialSearch() {
-    return app.current.constructor.providesInitialSearch && this.params().q;
+    return app.current.type.providesInitialSearch && this.params().q;
   }
 
   /**