Modularize file transforms and provide a "transforms" UI

Summary:
Ref T7707. Available transforms are currently relatively hard-coded and don't really have any support UI.

Modularize them so we can build some support UI.

This doesn't actually use any of the new stuff yet: I want to make a clean cutover once I fix the aspect ratio stuff so I can pick up a cachekey/URI change as a side effect.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T7707

Differential Revision: https://secure.phabricator.com/D12808