Rework public_document_url to rely on env

No more switching the public host based on the incoming request.
Unstick GovspeakHelper

Way too much coupling to other helper files and the request context
here. These helpers should function independantly of any controller
context or request objects.
Kill off Whitehall.public_hosts and other helpers

Final piece of the puzzle: we no longer need to map incoming request
hosts to public hosts - the public host is always the same and always
derived from the env.
Remove public_host helper

No need for this anymore - the API presenters are only ever used in a
front-end context so they should get the correct host via the existing
ENV.