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 don't think this docstring is accurate anymore. Maybe something along the lines of this would be better.
```
If the user accessed the third party auth flow while trying to enroll in a course,
automatically enroll them in that course if possible.
```