Removed precarious circular import with WidgyPage

There is a new `get_widgypage_model` method that mirrors
`django.contrib.auth.get_user_model`.

There are some backwards incompatible changes with this change, but we might as well get them out of the way.

-  `WIDGY_MEZZANINE_PAGE_MODEL` should look like `widgets.WidgyPage` now instead of `widgets.models.WidgyPage`.
-  `WidgyPageMixin` now lives in `widgy.contrib.widgy_mezzanine.models`