Add support for checking object permissions in PermissionRequiredMixin

[Tobias Bengfort]

As far as I understand, django's architecture supports object permissions, but external apps like django-rules or django-guardian are supposed to provide actual implementations. I think this approach is great. However, I believe that there are some key points where django core should provide more tooling so that the external apps can concentrate more on building the actual authentication backends.

On such point is PermissionRequiredMixin. If you want to use object permissions, you have to either replace or extend it.

A new method get_permission_object() could be added. This is what django-guardian does. However, in django core it should return None by default for backwards compatibility (In django-guardian it falls back to get_object()).

I sent a mail (​https://groups.google.com/forum/#!topic/django-developers/5vpc_DExhUw) in which I proposed this and some more changes to object permission support. Unfortunately, there was no response yet.

A patch is available at ​https://github.com/django/django/pull/10600