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.