Redirect instead of 400 from file on wrong domain

Summary: We recently opted for 'security.alternate-file-domain' and we have some hotlinks to the original domain.

Test Plan: Enabled 'security.alternate-file-domain', observed redirect.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3380