when creating a service, the api accepts a `service_domain` field that
it uses to populate the letter branding - if the service domain is
known to match an existing letter branding option, use that
automatically. However, the admin currently doesn't know about this
field yet so doesn't pass anything through - the api erroneously
searches the DB for letter branding with a domain of None - which they
currently all have.
This meant that when services were created, their letter branding was
set to the most recent row in the DB (that matched None).