CLC-3215 Streamline redirects and authentication

https://jira.ets.berkeley.edu/jira/browse/CLC-3215

Another try, this time dealing with Omniauth's ignoring of default_url_options.

Since I won't have time to work on this at the office, I've already got the revert branch ready. If reversion is needed, the temporary log message in the last commit will give us a little more info to work with.