This relationship is via the `Organisation` now; we don’t use this
column to fudge a relationship based on the user’s email address and the
matching something in these columns.
The NHS is a special case because it’s not one organisation, but it does
have one consistent brand. So anyone working for an NHS organisation
should have their default branding set when they create a service, even
if we know nothing about their specific organisation.
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).
However, until we can create a letter without a logo, we will still default to hm-government, because the dvla_organisation is set on the service.
This does simplify the code.
Also removed the inserts to letter_branding in the data migration file, because we can deploy this before the rest of the work is finished. But we will need to do it later.
