R1:9ba4f24e9304

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