Modify Album API endpoint to return a count attribute instead of a full assets array (#454)

* Change API to return assets count and change web behavior accordingly

* Refactor assets.length

* Explicitly declare type of assetCount so Dart SDK understand it

* Finished refactoring on mobile

mobile/lib/modules/album/views/album_viewer_page.dart

@@ -203,7 +203,7 @@ class AlbumViewerPage extends HookConsumerWidget {
                   assetList: albumInfo.assets,
                 );
               },
-              childCount: albumInfo.assets.length,
+              childCount: albumInfo.assetCount,
             ),
           ),
         );