UI improvements when deleting objects

Towards #79.

Shows nested child objects that will be deleted when deleting a single object or multiple objects.

I'm a bit unsure about this PR so any feedback would be welcomed. I couldn't figure out a nice way of getting tests into djadmin2 so they're part of the example app, which doesn't feel nice. They are working tests however.

The NestedObjects class and its tests are taken from the Django core. I've mentioned this in a comment in the code.