R1:332f4ab66d18

Restore support for using "arc download" to fetch files with no "security.alternate-file-domain"

Summary:
Fixes T13132. I removed this branch in D19156 when tightening the logic for the new CSP header, but there's a legitimate need for it: downloading files via `arc download`, or more generally being an API consumer of files.

This is not completely safe, but attacks I'm aware of (particularly, cookie fixation, where an attacker could potentially force a victim to become logged in to an account they…