Make the Pholio download button actually download

Summary:

• When the button is clicked, actually download the file or image.
• Add aural hints for the icon-only buttons.
• Use a "photo" icon for "view raw image", so the "arrows pointing outward" icon can be used for "fullscreen" some day.

Test Plan: Clicked link, got a download.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9574