Login/Registration A/B test cleanup

When the feature flag is enabled, replace the old login/registration pages with the new ones.  We can deploy this once the A/B test completes successfully.

* Update the old URLs to point to the new views (but don't delete the old views).
* Clean up as much of the third party auth stuff as possible, although we need to respect auth entry points from old sessions.

Reviewers: @stephensanchez @rlucioni 
I wonder if it's worth creating a dev story to remind us to come back and remove these old endpoints once and for all.