Name the client's region in every event, for the notifier's mail accounts

notifier can't tell which region a client is in from the events it gets; it asks identity-service before each send. To send through a mail account per region (ACM-296, whatever ACM-305 decides about Mailcrest), every event should say the client's region.

Acceptance criteria

• request.created, request.decided, reminder.due, notice.created and code.requested carry region.
• A test fails if an event is published without it.
• notifier stops asking identity-service for the region.