During the development of django-polymorphic-tree it was discovered that
the PolymorphicParentModelAdmin could actually be made much simpler.
It features a `child_models` attribute now, so there is very little code
needed to actually implement a polymorphic admin now.
Also found various issues which are together fixed in this commit for
pulling.
Extracted from django-fluent-pages, ready for other apps too.
The polymorphic admin is implemented via a parent admin for the base
model, and separate admin interfaces for the child models.
The parent model needs to inherit PolymorphicParentModelAdmin,
and override `get_admin_for_model()` and `get_child_model_classes()`
to find the child admin interfaces.
The derived models have their own `ModelAdmin` class, which inherits
from `PolymorphicChildModelAdmin`. The parent admin redirects it's
change and delete views to the child admin.
By adding `polymorphic` to the INSTALLED_APPS, the breadcrumbs will be
fixed as well, to remain unchanged between the child applications.
Diederik van der Boor