Prepare track selection page for new audit track

This is the platform-side work required to prepare for the new audit track (audit/honor track merge). @wedaly and @dianakhuang, could you please review this?

I also need some design input; I'll be speaking with @marcotuts to get it. I removed the `border-top` property from the CSS since it no longer served a purpose with the honor code dropdown removed. However, there are other issues, and there's nothing keeping us from using the styling used by the experimental branch.
Please leave a comment here that we can remove this line if the experimental condition wins in the auto-register A/B test.  Functionally, it doesn't matter, but it will avoid hitting the database.