Add reference to one off letters

Letters should always have a reference, because that’s what DVLA use to
tell us when they’ve sent a letter.

If a letter has a reference of `None` then DVLA say they’ve sent a
letter with a reference of `'None'`. This means we can never reconcile
the letter, which means it stays in `created`, which means it never
gets billed.

We don’t think this has affected any real letters yet, just ones that
we’ve sent as tests.

app/service/send_notification.py

@@ -1,5 +1,6 @@
 from sqlalchemy.orm.exc import NoResultFound
 
+from app import create_random_identifier
 from app.config import QueueNames
 from app.dao.notifications_dao import _update_notification_status
 from app.dao.service_email_reply_to_dao import dao_get_reply_to_by_id
@@ -37,6 +38,12 @@ def validate_created_by(service, created_by_id):
         raise BadRequestError(message=message)
 
 
+def create_one_off_reference(template_type):
+    if template_type != LETTER_TYPE:
+        return None
+    return create_random_identifier()
+
+
 def send_one_off_notification(service_id, post_data):
     service = dao_fetch_service_by_id(service_id)
     template = dao_get_template_by_id_and_service_id(
@@ -77,7 +84,8 @@ def send_one_off_notification(service_id, post_data):
         api_key_id=None,
         key_type=KEY_TYPE_NORMAL,
         created_by_id=post_data['created_by'],
-        reply_to_text=reply_to
+        reply_to_text=reply_to,
+        reference=create_one_off_reference(template.template_type),
     )
 
     queue_name = QueueNames.PRIORITY if template.process_type == PRIORITY else None