Edition::GovDelivery module for posting to gov delivery on publishing

Still need to modify Whitehall.gov_delivery_url to use Plek

Possible change to the content types, to use the display_type if too much info
Add mattr_accessor and rename module to GovUKDelivery

We can just use Plek to get the current hostname, as it'll be accessed
via the regular scheme.

Also renamed to GovUKDelivery, as that's the name of the
service (GovDelivery is the bit the service interacts with).
wip
rename file to be inline with module