Provide "Undo" on Pholio mock edits

Summary: Ref T2637. Allows you to "undo" if you delete an image from a mock by accident.

Test Plan:
Deleted; undo'd.

{F50878}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2637

Differential Revision: https://secure.phabricator.com/D6498