Allow Pholio mock images to be drag-reordered

Summary:
Ref T3640. JS part only, should give you a list in `imageOrder` on the server that you can read with `$request->getStrList('imageOrder')`.

NOTE: You can't drag images into the first position; this is an existing thing that I just need to fix with DraggableList.

@chad might have some design feedback.

Test Plan: Dragged images around, things seemed to work?

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3640

Differential Revision: https://secure.phabricator.com/D6729