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 a few hours.

• Viewed new module page.
• Grepped for all removed functions/constants.
• Viewed some transactions.
• Hovered over timestamps to get content source details.
• Added a comment via Conduit.
• Added a comment via web.
• Ran bin/storage upgrade --namespace XXXXX --no-quickstart -f to re-run all historic migrations.
• Generated some objects with bin/lipsum.
• Ran a bulk job on some tasks.
• Ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15521