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.
- Created TaskNames for DVLA_FILES rather than have DVLA_FILES in QueueNames
- Removed PROCESS_FTP from all_queues() as this was causing problems in picking up letter job tasks
- Created test to ensure that we don't arbitrarily add queue names to all_queues
