R1:21f07bf6f73b

Make Images in Pholio refer to mocks by PHID instead of ID

Summary:
Ref T11351. In Pholio, we currently use a `mockID`, but a `mockPHID` is generally preferable / more modern / more flexible. In particular, we need PHIDs to load handles and prefer PHIDs when exposing information to the API, and using PHIDs internally makes a bunch of things easier/better/faster and ~nothing harder/worse/slower.

I'll add some inlines about a few things.

Test Plan: Ran migrations, spot-checked database for sanity. Loaded…