Make set_as_broadcast_service use a single DB commit

We don't want things in a half state if there is an error during the
method. Therefore, we move it all into a single function that is wrapped
in a transaction.

Note, we copy the approach of
https://github.com/alphagov/notifications-api/blob/master/app/dao/services_dao.py#L293
by having a single new dao function that does all the DB work.

tests/app/service/test_rest.py

@@ -37,7 +37,6 @@ from app.models import (
     INBOUND_SMS_TYPE,
     NOTIFICATION_RETURNED_LETTER,
     UPLOAD_LETTERS,
-    ServiceBroadcastSettings,
 )
 from tests import create_authorization_header
 from tests.app.db import (