Third Party Auth Fixes

[ECOM-550](https://openedx.atlassian.net/browse/ECOM-550)
[ECOM-560](https://openedx.atlassian.net/browse/ECOM-560)

Reviewers: @dianakhuang @rlucioni 
FYI: @martynjames 

The basic idea is:
* Set the login cookie as a step in the authentication pipeline.
* Use query-string params to control auto-enrollment and to send users to the track selection page upon successful authentication.  The track selection page will handle the redirect to the dashboard if the user doesn't need to select a track (e.g. honor mode only)

The code is up in my sandbox (I'll send the link in chat).  I had to change some of the redirect logic on the course mode choose page, so I'd appreciate a close review of those changes.
I agree that approach would be more consistent.  This code is probably a little more paranoid than it needs to be, mainly because I'm worried about misconfiguration of the course modes in the database.  For example, I can imagine that a professional ed course might not have a minimum price set, in which case we still wouldn't want to let folks enroll.

I think once we create a proper UI for configuring course modes that enforces these constraints, we can make this method more general.