Semester data overhaul

[alex-shen1]

Status

• In Progress, and will be in progress for a while lol

GitHub Issues addressed

• This PR closes load_semester command should remove any sections that don't exist in the updated version of the data #529, closes Investigate missing instructors #461, closes Unify Instructor models with duplicate instances due to broken encoding #403

Seems like there's a discrepancy in the subjects available thru SIS and the ones we have. There's a lot more subject name discrepancies, but class code is probably what we actually care about (e.g. RELB is "Religion-Buddhism" in Lous/tCF but just Buddhism in SIS). This might be because I'm only doing undergrad classes right now, but it warrants more investigation.

Notes

• It seems like Lou's List was manually aggregating the subjects into departments, because the SIS API doesn't provide them. Accordingly, we might have to manually do this for other ones.
• Scraped every single undergrad class and it took this long:
  

[alex-shen1]

This will close #220 as well because it overhauls how we do semester data collection

Also #236, #467, #298