Does not work well with proxy classes

[MaybeThisIsRu]

When using with proxy classes, this package does not work well. Some improvements that can be made:

• Admin list display can refer to the proxy class instead of the parent class
• Admin list filter can refer to the proxy class instead of the parent class
• Documentation
  • After some monkey patching, I have realised that even though the package was telling me that the parent class was not registered with Moderation and that I should register it, there's no such need. We should be able to register all the proxy classes alone, without the parent class.
    • Registering both would break the app. Save handling or change/delete pages also break depending on which one you'd register (say, parent class) and which one you won't (say, proxy classes).

I'm unlikely to PR these changes yet as I'm testing them on my project internally.

[MaybeThisIsRu]

https://github.com/MaybeThisIsRu/django-moderation/tree/proxy-models

This seems to work, but a PR is not yet in the plan.