add more class metadata to the database

[aryarm]

While implementing #187, I realized that there was a lot of helpful course information from Hyperschedule that our database currently can not store.

1. course code
   this could be useful for figuring out whether we already imported a course via Hyperschedule, since some courses can have different course codes but the same name
2. course description
3. course instructor name(s) (related: see resource teacher attribute #128)
   Currently, we only store the course name. But we should consider storing this information in our database, too.

[aryarm]

The only challenge is that not all classes will have this information. Only the "course" classes will have it (ie the classes that represent physical courses and not departments or modules within a course).
So maybe the best way to do this is to actually create a separate table for "course" classes. And then create a foreign ID on the classes table that references the "course" it belongs to? The foreign ID can be null for classes that aren't "course" classes.