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