Undocumented assumption about custom user models

[kokolem]

The two-step activation workflow's RegistrationView expects the user object to have a email_user method:

django-registration/src/django_registration/backends/activation/views.py

Line 106 in 7a1d860

user.email_user(subject, message, settings.DEFAULT_FROM_EMAIL)

This is true for custom user models that inherit from AbstractUser, but not for those that inherit from AbstractBaseUser. I belive this should be documented in Compatibility of the built-in workflows with custom user models section.

[ubernostrum]

As noted over in #234, I'm already planning to do a new major version which will focus on the custom-user handling. So I'll look into fixing the documentation for now, but also the main focus is going to be on making the whole thing less brittle.