Add environment specific favicons

* `Pa` rather than `P` because of the clash with Publisher
Add test coverage for existing observer behaviour

The existing UpdateRouterObserver didn't have any unit tests. I've 
written some retrospectively so that it will be easier to change its
behaviour.
Don't delete routes for Whitehall artefacts

This changes the UpdateRouterObserver so that it no longer deletes 
routes for artefacts which are owned by Whitehall.

Whitehall manages deletion of its own routes, and in many cases we 
actually wish to maintain the routes so that the custom 'unpublishing' 
behaviour continues to work.

However, we should continue to support registering Whitehall routes
as Whitehall now relies on Panopticon to register routes for detailed 
guides.
Merge pull request #203 from alphagov/dont-delete-whitehall-routes

Don't delete routes for Whitehall artefacts
Clarify what is passed to favicon_link_tag
Merge pull request #201 from alphagov/favicons

Add environment specific favicons