Cancel notifications for group when re-showing chat

Signed-off-by: Mario Danic <mario@lovelyhq.com>

app/src/main/java/com/nextcloud/talk/controllers/ChatController.java

@@ -450,6 +450,10 @@ public class ChatController extends BaseController implements MessagesListAdapte
         if (getActivity() != null) {
             new KeyboardUtils(getActivity(), getView(), false);
         }
+
+        if (inChat) {
+            NotificationUtils.cancelExistingNotifications(getApplicationContext(), conversationUser);
+        }
     }
 
     @Override