Compare letter page count with billable units in DVLA response file

We compare the page_count in the response file we receive from the DVLA
with the billable_units of the letter. If these don't match, we log an
error.

app/dao/notifications_dao.py

@@ -471,6 +471,13 @@ def dao_get_notifications_by_to_field(service_id, search_term, statuses=None):
     return results
 
 
+@statsd(namespace="dao")
+def dao_get_notification_by_reference(reference):
+    return Notification.query.filter(
+        Notification.reference == reference
+    ).one()
+
+
 @statsd(namespace="dao")
 def dao_get_notifications_by_references(references):
     return Notification.query.filter(