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.

tests/app/main/views/test_activity.py

@@ -32,7 +32,7 @@ from tests.conftest import (
             '',
             [
                 'created', 'pending', 'sending', 'pending-virus-check',
-                'delivered', 'sent',
+                'delivered', 'sent', 'returned-letter',
                 'failed', 'temporary-failure', 'permanent-failure', 'technical-failure', 'virus-scan-failed',
             ]
         ),
@@ -42,7 +42,7 @@ from tests.conftest import (
         ),
         (
             'delivered',
-            ['delivered', 'sent']
+            ['delivered', 'sent', 'returned-letter']
         ),
         (
             'failed',