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