Add transparency to Pholio Edit thumbs

Summary: Makes the background transparent for uploaded thumbs. This page in general needs lots of work, but here's the minimum. Fixes T10986

Test Plan: Edit a Mock with a transparent jeff.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10986

Differential Revision: https://secure.phabricator.com/D15957