Add the owner_search_fields option to FolderAdmin.

In order to make it easier to work with custom User models, I have added
an owner_search_fields option to the FolderAdmin class, which

  1.  Allows users to declare which search fields they want.
  2.  Attempts to discover which fields are available on the User model
      and using those.

This makes it a lot easier to avoid errors when there are custom User
models.