Integrate django admin sites

This shares some commits with the branch `integrate_django_admin_groups`, as it depends on some of those changes.

Adds a 'Sites' menu item, with just a simple Site form at the moment, albeit with the fancy page chooser. There's no special validation or cleaning on `delete()`.

NB there won't be a permission field registered for add/change/delete Sites in the new groups menu yet, as that's on a parallel branch. That may need doing once these two are both merged into the main codebase.

This ought to resolve #21, among other issues discussed recently.