Adding service_id to the sort order for the letters being sent to print.

We have had a few instances where letters have caused problems. Particularly for precompiled letters, often the issue comes from the same service.
The hope is that by adding a sort order this will help the print provider narrow down the problem.

There is a small degradation of the performance of the query, but it's not enough to concern me.

app/dao/notifications_dao.py

@@ -728,6 +728,7 @@ def dao_get_letters_to_be_printed(print_run_deadline, postage):
         Notification.key_type == KEY_TYPE_NORMAL,
         Notification.postage == postage,
     ).order_by(
+        Notification.service_id,
         Notification.created_at
     ).all()
     return notifications