Send 403 for admin pages without being admin
Summary: I've also moved the response generation for 404 from
AphrontDefaultApplicationConfiguration to buildResponseString()
Test Plan:
Visit /
Visit /mail/
Visit /x/
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, epriestley, vrana
Differential Revision: https://secure.phabricator.com/D1406