- uses the reference field on the notifications table to store a 16char random string used to cross reference DVLA letters back to the notification
- used as letter barcode does not have space for a UUID notification id
Depends on https://github.com/alphagov/notifications-utils/pull/149
Renamed the numeric_id to notification_reference in utils and changed validation rules to match this
Note also the persist_notification method set "reference" to be "client_reference" which is confusing and they are different things, so fixed this too.