Explicitly cast notification ids to integers when using them for setting maxId

This fixes the bug where BE receives incorrect `since_id` when marking
notifications as read.

src/modules/statuses.js

@@ -273,10 +273,11 @@ const addNewNotifications = (state, { dispatch, notifications, older, visibleNot
 
     // Only add a new notification if we don't have one for the same action
     if (!state.notifications.idStore.hasOwnProperty(notification.id)) {
-      state.notifications.maxId = notification.id > state.notifications.maxId
+      const notificationId = parseInt(notification.id, 10)
+      state.notifications.maxId = notificationId > parseInt(state.notifications.maxId, 10)
         ? notification.id
         : state.notifications.maxId
-      state.notifications.minId = notification.id < state.notifications.minId
+      state.notifications.minId = notificationId < parseInt(state.notifications.minId, 10)
         ? notification.id
         : state.notifications.minId