Escape user-supplied content when generating HTML

Also gets rid of the ERB template in favour of straight-forward
string-interpolation.
GovUkDeliveryEndPoint delays notification

Instead of performing the notification in-bound, we queue a delayed
job to do the work later.
Merge pull request #472 from alphagov/async-govdelivery

Async govdelivery

https://www.pivotaltracker.com/story/show/47714397