0d5f2fd943
As discovered in django-polymorphic-tree and django-fluent-pages, the raw_id_fields didn't work in Django 1.4 because the fields actively check which models are actually registered in the admin site. Hence, the parent admin site _registry is inserted in the child admin as well. This also completely moves the initialisation of the child admin into this class, using a `get_child_models()` function, akin to the static `child_models` attribute.