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).