R1:b3faa2874cc3

Restore a Mock key to Pholio Images

Summary: Ref T11351. We only query for images by PHID or by Mock, so the only key we need for now is `<mockPHID>`.

Test Plan: Ran `bin/storage upgrade`.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19934