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.