R1:a2fdf14275f9

Stop using forms to download files in file embed and lightbox elements

Summary: Depends on D19156. Ref T13094. This replaces the remaining forms in the file embed view and lightbox with normal download links.

Test Plan: Clicked "Download" and lightbox -> download for embedded files.

Maniphest Tasks: T13094

Differential Revision: https://secure.phabricator.com/D19157