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
WIP
announcements type checking
publicationesque.rb should define search_format_types
publication gov_uk_delivery callback
remove debugging
add in notify_govuk_delivery expectations
Only include the module where its needed
remove redundant gov_delivery_url
simplify the Edition::GovUkDelivery module

removing farrady code to be replaced with gds api adapters
Split out listing of applicable tags
Make tag URLs absolute

This required adding some properties to the global Whitehall object. To
be honest I'm surprised we've never needed to do this before.
Only try and notify govuk_delivery if there are tags
Correct bad value for APP_DOMAIN

This is meant to be an actual domain, not a URL.
Add tests and real GdsApiAdapter code
Change unit test to just check for invocation
Rename initializer to match class