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 newly implicit
protocol (derived from the environment) and the previously explicit one
hardcoded into the source.
DRY up url generation of atom feeds
Get rid of unused methods
Use consistent domain for test env
Don't special case protocol scheme

Once again, Plek tells us the truth.
Remove redundant code

This is no longer relevant
Remove more redundant code

Direct route registration is no longer performed - route registration
happens during panopticon registration now.
Remove more redundant code

This class was used in a data migration that was run long ago and is no
longer needed
Remove lots of hardcoded URLs

This also gets rid of some special case tests that were testing the
difference between the public and admin hosts on preview and production.
As we are now solely depending on the truth as told by Plek, we don't
need to have separate tests for preview and production in most cases.