remove emailbackend, improve tests, add custom user docs

docs/email-authentication.md

media/django-admin-login-page.png

media/django-admin-user-detail.png

media/django-admin-user-list.png

oser_cs/oser_cs/settings/default.py

oser_cs/tests/utils.py

oser_cs/users/admin.py

oser_cs/users/backends.py

oser_cs/users/models.py

oser_cs/users/tests.py