Make SMS prefix setting non-nullable

We have now:
- defaulted new services to start with this column set to `true`
- migrated all preexisting services[1] to have either `true` or `false` set
  for this column

There is no way for a service to switch back from `true`/`false` to
`null`.

This means that we can safely enforce a non-nullable constraint on this
column now.

1. There is a little gotcha: the GOV.UK Notify service still relies on
   the `sms_sender` column. It doesn’t have a row in the
   `service_sms_senders` table. This means the previous migration
   never changed this service’s value for `prefix_sms` from `null`. So
   this commit also changes its value to `False`, so that the rest of
   the migration, of the whole column, is possible.

tests/app/conftest.py

@@ -992,7 +992,7 @@ def notify_service(notify_db, notify_db_session):
             'active': True,
             'restricted': False,
             'email_from': 'notify.service',
-            'created_by': user
+            'created_by': user,
         }
         service = Service(**data)
         db.session.add(service)