Feature - Implemented virtual scroll on web (#573)

This PR implemented a virtual scroll on the web, as seen in this article.

[Building the Google Photos Web UI](https://medium.com/google-design/google-photos-45b714dfbed1)

mobile/openapi/test/asset_count_by_time_bucket_test.dart

@@ -0,0 +1,32 @@
+//
+// AUTO-GENERATED FILE, DO NOT MODIFY!
+//
+// @dart=2.12
+
+// ignore_for_file: unused_element, unused_import
+// ignore_for_file: always_put_required_named_parameters_first
+// ignore_for_file: constant_identifier_names
+// ignore_for_file: lines_longer_than_80_chars
+
+import 'package:openapi/api.dart';
+import 'package:test/test.dart';
+
+// tests for AssetCountByTimeBucket
+void main() {
+  // final instance = AssetCountByTimeBucket();
+
+  group('test AssetCountByTimeBucket', () {
+    // String timeBucket
+    test('to test the property `timeBucket`', () async {
+      // TODO
+    });
+
+    // int count
+    test('to test the property `count`', () async {
+      // TODO
+    });
+
+
+  });
+
+}