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