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.
Why are we adding an `unsupported` mode?