R1:601aaa5a86c1

Modularize content sources

Summary:
Ref T10537. For Nuance, I want to introduce new sources (like "GitHub" or "GitHub via Nuance" or something) but this needs to modularize eventually.

Split ContentSource apart so applications can add new content sources.

Test Plan:
This change has huge surface area, so I'll hold it until post-release. I think it's fairly safe (and if it does break anything, the breaks should be fatals, not anything subtle or difficult to fix), there's just no reason not to hold it for…