Simplify and tidy before extraction
Pull gov uk notification to background job.

This pulls the work for delivering the notification call out to a
background job. Because the notification call is now being made out-
of-bound from the request, we no longer need to rescue exceptions and
instead want to let the delayed job queue do its thing and retry
failed attempts.
Pull email body code out to job

This method didn't feel like it belonged in the editions mixin. Better
to put it nearer to the point of use. I'm also tempted to pull out the
govuk_delivery_tags method, but this will do for now.
Gov deliver notifactions should expect 400s

A 400 response is returned when there are no subscribers for a given
notification call, so we need to handle them to avoid needlessly
retrying.
Escape HTML characters in gov delivery email body
Tidy up code and tests post-rebase