Add a task to process returned letter lists

Adds an API endpoint `/letters/returned` that accepts a list of
notification references and creates a task to update their status.

Adds a new task that uses the list of references to update the status
of notifications to 'returned-letter'.

The update is currently done using a single query and logs the
number of changed records (including notification history records).
This could potentially be done within the `/letters/returned` endpoint,
but creating a job right away allows us to extend this more easily
in the future (e.g. logging missing notifications or adding callbacks).

The job is using the database tasks queue.

app/models.py

@@ -1309,7 +1309,7 @@ class Notification(db.Model):
 
         if self.status in [NOTIFICATION_CREATED, NOTIFICATION_SENDING]:
             return NOTIFICATION_STATUS_LETTER_ACCEPTED
-        elif self.status == NOTIFICATION_DELIVERED:
+        elif self.status in [NOTIFICATION_DELIVERED, NOTIFICATION_RETURNED_LETTER]:
             return NOTIFICATION_STATUS_LETTER_RECEIVED
         else:
             # Currently can only be technical-failure