Fix regression in generic signal handling in Django 1.7

This un-breaks Mezzanine under Django 1.7. (see #1039)

1.7's app loading refactor changes the signature of ``django.db.models.get_model()``, which is now a proxy for ``Apps.get_model()`` and has no ``seed_cache`` or ``only_installed`` parameters – this functionality is now provided by ``Apps.get_registered_model()`` instead.

The function also now raises an exception where previously it returned None. I've written a wrapper function to emulate the 1.7 behaviour under older versions of Django.