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 suspect that the cleaner way of doing this is to check whether or not there are any available modes that do not have a minimum price attached to them. This is fine for now since we have to get it in, but I think we should consider moving over towards thinking of modes that way.