R1:2bdb5404c7e9

Implement new profile transform with amazing "error handling" feature

Summary:
Ref T7707. Ref T4406. Ref T2479. This implements the profile-style (fixed width and height) transforms in a modern way.

- Added a "regnerate" feature to the support UI to make testing easier and surface errors.
- Laboriously check errors from everything.
- Fix the profile thumbnailing so it crops properly instead of leaving margins.
- Also defuses the "gigantic white PNG" attack.

This doesn't handle the imagemagick…