CC DVLA in tickets about outstanding letters

Previously we sent them emails about this manually. We also tried
a Zendesk macro/trigger approach, but using a CC means:

- We can control the behaviour ourselves (Zendesk triggers can only
be edited by admins outside our team).

- We keep the DVLA notification approach consistent and in one place,
so notifications always go to the same people.

- Any further (public) updates to the ticket will also trigger a
notification to DVLA (previous trigger only notified on creation).

tests/app/celery/test_nightly_tasks.py

@@ -250,6 +250,7 @@ def test_create_ticket_if_letter_notifications_still_sending(notify_api, mocker)
     mock_create_ticket.assert_called_once_with(
         ANY,
         subject='[test] Letters still sending',
+        email_ccs=current_app.config['DVLA_EMAIL_ADDRESSES'],
         message=(
             "There are 1 letters in the 'sending' state from Monday 15 January. Resolve using "
             "https://github.com/alphagov/notifications-manuals/wiki/Support-Runbook#deal-with-letters-still-in-sending"