When a user creates a service we can take a pretty good guess at what
organisation they’re from.
For many organisations, especially local councils, GOV.UK branding is
not appropriate for their service. But right now every service:
- gets created with GOV.UK branding
- has to ask us to change it, even if they’ve already done so for other
services they run
This commit starts using the `domain` field on the email branding table
to lookup what email branding to assign to a service automatically,
where we’re sure there’s a sensible default.