nav: Add a button for MentionsNarrow.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/app.dart

@@ -276,6 +276,12 @@ class HomePage extends StatelessWidget {
                 narrow: const CombinedFeedNarrow())),
             child: Text(zulipLocalizations.combinedFeedPageTitle)),
           const SizedBox(height: 16),
+          ElevatedButton(
+            onPressed: () => Navigator.push(context,
+              MessageListPage.buildRoute(context: context,
+                narrow: const MentionsNarrow())),
+            child: Text(zulipLocalizations.mentionsPageTitle)),
+          const SizedBox(height: 16),
           ElevatedButton(
             onPressed: () => Navigator.push(context,
               InboxPage.buildRoute(context: context)),