Decouple subscribers from publishing services

The edition service objects should not need to have knowledge of the
callbacks/subscribers that are interested in their outcome. Instead,
we rely on another object to coordinate the subscription and
notification of interested parties - The Edition::ServiceCoordinator.
This uses the simple queue pub/sub class from within ActiveSupport to
push events back and forth.
SRP: Untangle GovUk Delivery from Edition classes

Edition no longer needs any awareness of GovUK delivery notifications.