utilize built-in views from django.contrib.auth

Use the built-in `auth` system for Django 1.7 to handle login, instead of rewriting the logic. The logic for handling signing in/out and user registration is already well-written in the Django source, with CSRF protection and disabling caching. I doubt there is any need to alter the built-in auth system.

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[User authentication in Django](https://docs.djangoproject.com/en/1.7/topics/auth/)
[Source code for `django.contrib.auth.views`](https://docs.djangoproject.com/en/1.7/_modules/django/contrib/auth/views/)