Default new services to prefixing text messages

We want new services, when they do the tour, to see how the service name
they just made shows up in the messages. This is how it (should) work
at the moment (although got broken because of the multiple senders
stuff).

Need to do this before we do the migration now otherwise a new service
could sneak in with this setting still set to `null`.

tests/app/dao/test_services_dao.py

@@ -93,6 +93,7 @@ def test_create_service(sample_user):
     assert service_db.branding == BRANDING_GOVUK
     assert service_db.dvla_organisation_id == DVLA_ORG_HM_GOVERNMENT
     assert service_db.research_mode is False
+    assert service_db.prefix_sms is True
     assert service.active is True
     assert sample_user in service_db.users