Allow only CDN routes when using security.alternate-file-domain

Summary:
Instead of allowing all routes based on security.alternate-file-domain, now, when security.alternate-file-domain is set, and the request matches this domain, requests are validated against an explicit list.  Allowed routes:
-  /res/
-  /file/data/
-  /file/xform/
-  /phame/r/

This will be redone by T5702 to be less of a hack.

Test Plan:
- browse around (incl. Phame live) to make sure there is no regression from this when security.alternate-file-domain is not used.
- check that celerity resources and files (incl. previews) are served with security.alternate-file-domain set.
- check that phame live blog is serving its css correctly with security.alternate-file-domain set.
- check that requests outside of the whitelist generate an exception for security.alternate-file-domain

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10048