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.
This isn't a new mode. It's there for the purposes of testing the views' behavior when someone requests a mode not included in a course's `allowed_modes`. You could do this by changing the value of the `name` attribute on a track selection button before submitting your choice.