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).

app/celery/nightly_tasks.py

@@ -185,6 +185,7 @@ def raise_alert_if_letter_notifications_still_sending():
 
             ticket = NotifySupportTicket(
                 subject=f"[{current_app.config['NOTIFY_ENVIRONMENT']}] Letters still sending",
+                email_ccs=current_app.config['DVLA_EMAIL_ADDRESSES'],
                 message=message,
                 ticket_type=NotifySupportTicket.TYPE_INCIDENT,
                 technical_ticket=True,