Decouple YesodAuthPersist from Persistent

Hi!  This is the change described in <https://groups.google.com/d/msg/yesodweb/HDd01ymvE5Y/yMRk9UDSF-sJ>.

I had some trouble getting it tested.  What’s the preferred way to get development versions of the Yesod ecosystem building/running, by the way?

The versions of the scaffolded site that use Persistent will need an extra `instance YesodAuthPersist App` in `Foundation.hs` if this goes in.