Expose factory filters in the controller/view

[joshfrench]

For extensions that may want to restrict factory selection by page class, page factory, current user role, &c. Will involve: exposing @page instance in factory_link controller; adding a factories method (naming?) which provides all Factory classes by default; documenting how to alias/override this method.

[joshfrench]

Expose factory list for refinement by other extensions (closed by b12db50)