move hard-coded login form variables to settings.py

[dongourley]

So this can be more easily used by other consortia, need to be able to define the institutions in the consortium (currently hard-coded in templates/login.html) and the URL to send the user to login (currently hard-coded in static/js/frontend.js) to settings.py.

Also, need a setting in settings.py to turn on and off the backdoor login that is defined in fines_app.py.

Update README.md when these settings are implemented.