At login view, sanity-check settings and warn accordingly

[Liz4v]

This project integrates pretty tightly with default Django login infrastructure, which means there's a number of settings that must be present in the settings file. Things to check:

1. Do we have any of the appropriate AUTHENTICATION_BACKENDS?
2. If e-mail login is enabled, do we have both authentication and e-mail backends set?

See existing code in app.py for guidance.

This is a great first pull request if you want to join this project. I have no plans of implementing this myself.