New setting pointing to custom Model

[erikcw]

The built in Signals (in models.py) don’t work with a custom Hook model because they are hard coded to use the built in model.

Seems like the right way to handle this is to add an optional setting pointing to the custom model, and then use the value of that setting instead of hardcoded references to models.Hook.

[avelis]

@erikcw If you have time to open a PR with this I would be happy to take a look and merge it in.

[imposeren]

Merged but not published on PyPI