Make clicking on file icon in lightbox download file

Summary: Ref T3612, this adds a anchor around the large icon with hover state so you can download from here as well.

Test Plan: Hover over .ics file, click, get download.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3612

Differential Revision: https://secure.phabricator.com/D16977