Give Phame blogs an explicit 404 controller
Summary:
Ref T11076. Ref T9897. Bad links on Phame blogs are currently made worse because we try to prompt you to login on a non-cookie domain.
Instead, just 404 in a vanilla way. Do so cleanly on external domains.
Test Plan: {F1672399}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9897, T11076
Differential Revision: https://secure.phabricator.com/D16010