When viewing raw file content in Differential, cache it into the File tool before displaying it

Summary:
@alok reported a vulnerability where Flash will run carefully-crafted plain text
files.

When the user requests a raw file, cache it into Files if it isn't already
there. Then redirect them to Files. This solves the problem by executing the
SWF/TXT with CDN-domain permissions, not content-domain permissions, provided
the install is correctly configured. (Followup diff coming to make this more
universally true.)

Test Plan: Clicked "View Old File", "View New File" in an alt-domain
configuration, got redirected to a cookie-free domain before being delivered the
response.

Reviewers: btrahan, alok

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1607