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 case (for animated GIFs) yet.

• Uploaded a variety of wide/narrow/small/large files and converted them into sensible profile pictures.
• Tried to thumbnail some text files.
• Set the pixel-size and file-size limits artificially small and hit them.
• Used "regenerate" a bunch while testing the rest of this stuff.
• Verified that non-regenerate flows still produce a default/placeholder image.