PolicyGroup is not searchable, PolicyAdvisoryGroup and PolicyTeam are

We make that clearer by implementing search_link to raise a NotImplementedError on PolicyGroup.
Tidy up and identify all route generating helpers

Moved some from ApplicationHelper to a XXXRoutesHelper and renamed ClassificationHelper because it's only about routes.
Add Whitehall::UrlMaker as an object for getting urls

Not entirely happy with the testing of this object (just asserting it has the right ancestors).  Will make sure those ancestors have better test coverage in a later commit.
Implement a no-op params method on UrlMaker

This is for the changes that localised_url_helper makes to the localized routes to look in params[:locale] before manipulating the url it returns (e.g. to preserve the locale we are already in).
Stop including url_helpers in models and use Whitehall.url_maker

Including all the routing methods in all these models is unnecessary, and we aren't even guaranteed that including the rails url_helpers is enough because we don't have any overwrites that our own helpers might produce.
Include the admin editions route helpers in the url maker

And include some tests for that helper (so we can be confident that url maker is doing the right thing).
Make PublicDocumentRoutesHelper#model_name a public method

Also give it a more specific name.  This is so we can use it elsewhere.
Replace routes_helper implementations with url_maker versions
Places that include routes and admin helpers now use url_maker

DocumentMappings is what required the model_name_for_route_recognition method to be made public, because we need to do what document_url does, but without letting it override id: with the edition slug (document mappings sometimes wants to use the unpublishing.slug instead).