Added a new DAO function which archives letter contact blocks by
setting archived to True. This raises an ArchiveValidationError if
trying to archive the default letter block for a service or the default
letter contact block for a template.
Added a new endpoint for archiving letter contact blocks.
Added a new boolean column, `archived`, with a default of False to the
three models which are used to specify the 'reply to' address for
notifications:
* ServiceEmailReplyTo
* ServiceSmsSender
* ServiceLetterContact
Added the GET /<uuid:service_id>/letter-contact/<uuid:letter_contact_id>
endpoint for returning a letter contact for a service with the letter contact id
Added the DAO for returning a letter contact by id
Added the GET '/<uuid:service_id>/letter-contact' endpoint for returning
all letter contact blocks for a service.
Updated the DAO for service letter contacts to return the default letter
contact first.