Write Custom AUTHENTICATION_BACKEND for PyLTI13

[jonespm]

PyLTI13 currently doesn't have a AUTHENTICATION_BACKEND that can be used. So we have to login and use the ModelBackend as a fallback.

If it had it's own backend similar to these projects we could just use this rather than storing the users locally.

The negative of this would be that OAuth keys are stored per user so this might complicate things with OAuth unless we were storing the users in a database locally.

It's possible that this may not be worthwhile for this project because of OAuth, or be more difficult.

This was a sample from one LTI project. https://github.com/wachjose88/django-lti-provider-auth/blob/master/lti_provider/backends.py
Here was another: https://github.com/ccnmtl/django-lti-provider/blob/master/lti_provider/auth.py