Incompatible with custom user model

[DgRosa]

Hi,

I'm testing your package on a personal project and I got the following exception when running the migrations.

I dig a little bit in the codebase and discovered the root of the problem, in the file models file you are importing the User model directly from django.contrib.auth.models import Group, User, where you should use get_user_model to select the set User model. You can know more about in the official docs.

Let me know if you need help.

Thank you for contribution.

[matteius]

Let me know if you want to open a fix PR and I can get it merged/shipped -- otherwise I will do so soon.

FWIW I have some time slated to use this on an upcoming project in the next month, assuming that still happens -- this will be seeing a lot more TLC.