With the introduction of new AdminSite and ModelAdmin classes for

our new administration UI, it's important that we set a precedent of

using these new classes for all model registration, even though the new

versions are compatible with the old ones.
This change switches all model registration to use our new AdminSite

explicitly, and switches all ModelAdmin subclasses to use our new base

class.

Unit tests pass.
Verified that these were registered correctly in the admin UI.