Added the random string reference to the letter

- uses the reference field on the notifications table to store a 16char random string used to cross reference DVLA letters back to the notification
- used as letter barcode does not have space for a UUID notification id

Depends on https://github.com/alphagov/notifications-utils/pull/149

Renamed the numeric_id to notification_reference in utils and changed validation rules to match this

Note also the persist_notification method set "reference" to be "client_reference" which is confusing and they are different things, so fixed this too.

app/notifications/process_notifications.py

@@ -36,6 +36,7 @@ def persist_notification(template_id,
                          job_id=None,
                          job_row_number=None,
                          reference=None,
+                         client_reference=None,
                          notification_id=None,
                          simulated=False):
     # if simulated create a Notification model to return but do not persist the Notification to the dB
@@ -53,7 +54,8 @@ def persist_notification(template_id,
         created_at=created_at or datetime.utcnow(),
         job_id=job_id,
         job_row_number=job_row_number,
-        client_reference=reference
+        client_reference=client_reference,
+        reference=reference
     )
     if not simulated:
         dao_create_notification(notification)