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 Pholio, saw data unchanged. Created and edited images.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19914