Primary listing

[Josh-Cena]

To address https://github.com/coursetable/coursetable/issues/1459, we need to have the concept of a "primary listing" so when we collapse all cross-listings into a dropdown we can display a course code and a "+n" suffix.

This data is already available at https://developers.yale.edu/courseswebservicev3. My vision is that we should migrate as much information as possible to this API, and source the rest from YCS.