Move ALL files to serve from the alternate file domain, not just files without "Content-Disposition: attachment"

Summary:
We currently serve some files off the primary domain (with "Content-Disposition:
attachment" + a CSRF check) and some files off the alternate domain (without
either).

This is not sufficient, because some UAs (like the iPad) ignore
"Content-Disposition: attachment". So there's an attack that goes like this:

• Alice uploads xss.html
• Alice says to Bob "hey download this file on your iPad"
  • Bob clicks "Download" on Phabricator on his iPad, gets XSS'd.

(Because files are now authenticated only by knowing the PHID and secret key,
this also fixes the "no profile pictures in public feed while logged out"
issue.)

Test Plan: Viewed, info'd, and downloaded files

Reviewers: btrahan, arice, alok

Reviewed By: arice

CC: aran, epriestley

Maniphest Tasks: T843

Differential Revision: https://secure.phabricator.com/D1608