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

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -466,22 +466,6 @@ class AphrontDefaultApplicationConfiguration
               'redirect' => $response->getURI(),
             ));
       }
-    } else if ($response instanceof Aphront404Response) {
-
-      $failure = new AphrontRequestFailureView();
-      $failure->setHeader('404 Not Found');
-      $failure->appendChild(
-        '<p>The page you requested was not found.</p>');
-
-      $view = new PhabricatorStandardPageView();
-      $view->setTitle('404 Not Found');
-      $view->setRequest($this->getRequest());
-      $view->appendChild($failure);
-
-      $response = new AphrontWebpageResponse();
-      $response->setContent($view->render());
-      $response->setHTTPResponseCode(404);
-      return $response;
     }
 
     return $response;