Make Pholio work a little better on mobile

Summary:
Ref T2644. This adjusts thumb sizing so the "X" button is visible, and hides the uploader on devices for now.

The thumb stuff I'm sort of hacking (we'll cut off a little bit of wide thumbs on the iPhone), but it looks fine, is usable, and works a little better in landscape mode and at tablet sizes.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2644

Differential Revision: https://secure.phabricator.com/D9562