Correct misaligned django-cas integration

This PR fixes the django-cas integration so that it is inline with other authentication methods. It moves the configuration into the aws.py settings files and adds the integration to cms/studio instead of just being in LMS.  It also adds CAS auth redirection to the right login view in LMS.

@sarina with heads up to @ichuang @pdpinch @jbau and @johncox-google