Don't delete routes for Whitehall artefacts

https://www.pivotaltracker.com/story/show/77730146

At the moment, Panopticon is deleting routes from the router for any artefact which is archived. This includes Whitehall artefacts which have been "unpublished". This causes issues because "unpublished" Whitehall content often has a custom redirect or "gone" page which specifies the reason for unpublishing. When Panopticon deletes the route, the Router serves its own generic "gone" page instead.

This pull request changes the observer which interacts with the Router API, changing it so that it no longer deletes routes for artefacts which are owned by Whitehall. It also adds new unit test coverage for the observer.