Add a platform admin page to submit returned letter references

A platform admin form accepts a list of references (one per line)
received from DVLA and sends them to the API to update notification
statuses.

References we get from DVLA start with `NOTIFY00\d`, which isn't
part of the reference we store in the database, so we remove them
before sending the data to the API.

The new `returned-letter` status should be treated as `delivered`
for now until we decide a way to display returned letters to users.

app/__init__.py

@@ -385,6 +385,7 @@ def format_notification_status(status, template_type):
             'sent': 'Delivered',
             'pending-virus-check': 'Pending virus check',
             'virus-scan-failed': 'Virus detected',
+            'returned-letter': 'Delivered',
         }
     }[template_type].get(status, status)
 
@@ -410,6 +411,7 @@ def format_notification_status_as_field_status(status, notification_type):
             'accepted': None,
             'pending-virus-check': None,
             'virus-scan-failed': 'error',
+            'returned-letter': None,
         }
     }.get(
         notification_type,