Tidy up DataHygiene::PublishingApiRepublisher

Whitehall::PublishingApi.republish no longer returns anything significant and
instead raises an exception if something that isn't publishable is passed in.
Include "rendering_app" in unpublishing schema
Make tests more robust

Test for markup equivalence, not string match to deal with libxml
inconsistencies.
Tweak test data

An explanation shouldn't be present on a redirect unpublishing.