Set marketing site logged in cookie from third party auth.

[ECOM-550](https://openedx.atlassian.net/browse/ECOM-550): 3rd Part Auth - Header - Log In and Registration button showing after successful login

I'm going to run through the full 3rd party auth manual test plan on Monday, including edge cases like the user deleting just the logged in cookie at various points in the pipeline.  But I think this is far enough along to go into code review.

Reviewers: @dianakhuang @rlucioni 
FYI: @stephensanchez enjoy your vacation!