Process responses for letters even after the notification has been deleted.

This will continue to update the notification history for letter notifications.
We currently have an issue where the responses to letters from the provider is taking a long time.
This is due to the manual nature of their process.
Updating the status of the letter will still work if the notification has been purged.

Also turned back on the purge letter notification scheduled task.

app/dao/notifications_dao.py

@@ -463,6 +463,13 @@ def dao_get_notification_by_reference(reference):
     ).one()
 
 
+@statsd(namespace="dao")
+def dao_get_notification_history_by_reference(reference):
+    return NotificationHistory.query.filter(
+        NotificationHistory.reference == reference
+    ).one()
+
+
 @statsd(namespace="dao")
 def dao_get_notifications_by_references(references):
     return Notification.query.filter(