R1:c006cca9b1e9

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.