With the {F...} syntax, render non-images as links

Summary:
We render a huge picture of a PDF for PDFs right now, etc. This is hella dumb.

Also allow users to force this rendering style, and change the link name.

Test Plan: Uploaded image and non-image files, used layout=link and name=....

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T1040

Differential Revision: https://secure.phabricator.com/D2006