CLC-3215 Streamline redirects and authentication

https://jira.ets.berkeley.edu/jira/browse/CLC-3215

This PR attempts to solve two problems:

1. On a production server behind Apache, our redirects to a relative path have actually been redirecting twice: first to "http://PATH", and then to "https://PATH".

2. When we want to authenticate through CAS, instead of just redirecting to "/auth/cas" we've done an intervening redirect to "/login".

Fixing these might also enable deep-linking, but will at least reduce the noise.