WIP: Combined Login/Registration: Set up basic enrollment infrastructure.

With the third party auth fixes, we should be able to send everyone to the track selection page, and it will handle the redirects correctly.

There's one important exception (which I only realized this morning).  For white-label courses, we (1) add the course to the shopping cart, and (2) redirect to the shopping cart view page.  I'm not sure the best way to handle this given our current architecture.