R1:7e365eb8ae8c

Convert "profile" image transforms to the new pathway

Summary:
Ref T7707. This ends up being sort of complicated: to support 100x100 images in T4406, we need to scale small images //up// so they look OK when we scale them back down with `background-size` in CSS.

The rest of it is mostly straightforward.

Test Plan:
- Did an OAuth handshake and saw a scaled-up, scaled-down profile picture that looked correct.
- Used Pholio, edited pholio, embedded pholio.
- Uploaded a bunch of small/weird/big images…