Add download link to embedded files

Summary: Ref T3612. Doesn't render correctly, need help please. Adds a download icon into the renderfilelinkview to allow easier downloads.

Test Plan: Click on link, get download, click on file, get lightbox.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3612

Differential Revision: https://secure.phabricator.com/D16980