Whitehall.public_host derived from Plek

Up to now, we've set Whitehall.public_host in an initializer that is
overridden per-environment during deploy. Plek is the source of all
truth so we do not need to do this.
Remove more hardcoded URLs

In most cases, the default Whitehall.url_maker will give us what we need
to generate both public and admin urls. Some tests required slight
adjustment to reflect the slight difference in the implicit protocol
based on the environment and the previously explicit one hardcoded into
the source.
DRY up url generation of atom feeds