Fix a policy issue where permissions were not properly checked when disabling global builtin queries

Summary: See <https://hackerone.com/reports/1573143>. The pathway for disabling global builtin queries is missing a policy check. Add it.

Test Plan:
  - Accessed the "/search/delete/id/.../" URI for a global builtin query as a non-administrator.
  - Before patch: could improperly disable queries.
   -After patch: proper policy exception.

Differential Revision: https://secure.phabricator.com/D21851

src/applications/search/controller/PhabricatorSearchDeleteController.php

@@ -42,6 +42,19 @@ final class PhabricatorSearchDeleteController
       }
 
       $named_query = $engine->getBuiltinQuery($key);
+
+      // After loading a global query, make sure the viewer actually has
+      // permission to view and edit it.
+
+      PhabricatorPolicyFilter::requireCapability(
+        $viewer,
+        $named_query,
+        PhabricatorPolicyCapability::CAN_VIEW);
+
+      PhabricatorPolicyFilter::requireCapability(
+        $viewer,
+        $named_query,
+        PhabricatorPolicyCapability::CAN_EDIT);
     }
 
     $builtin = null;