Consolidate all of the page_model pluralization into a page_collection_name helper.

This also removes the `update_pages_path` helper in favor of a polymorphic path.

@jcoyne I found a few instances where we were need the singularized version.  Also the symbol vs. string differences in using inflections made the few lines where we would have to singularize exceptionally nasty looking.

So instead I have changed the return value of the helper to a symbol and added a new helper that pluralizes the `page_model` and use that around the application where necessary.  I suppose I could have done the reverse (have the controllers override the pluralized version and then the helper singularizes that) but I'm not sure there is a difference in approach.