This makes it easier to make a ModelBackend-compliant (with regards to
permissions) User model.
Thanks to cdestigter for the report about the relationship between
ModelBackend and permissions, and to the many users on django-dev that
contributed to the discussion about mixins.
Backport of 47e1df896b17aaaa97b73ef64010a7df4ea3d8d6 from master.