Add "course" field for books

[tbkissinger]

Would have the ability to post/search by course, would encourage more standardized book postings

[fatlotus]

Since every listing can show up in multiple categories, I wonder if there's a way to generalize the course name to apply to different types of things.

Maybe autocomplete for common course numbers? Core classes seem to use names where science classes use numbers — it might be tricky to wedge all of them into a shared schema.