Allow custom fields to be ordered ascending or descending

Summary:
Fixes T6864. This creates a sort of busy menu but I think that's proably fine -- users are opting into activating these fields for search anyway.

In the future, we could refine this as, e.g.:

  - don't show these options in the dropdown;
  - do show them on some new "http prefilling" sort of page;
  - then you access them as an advanced user with `?order=secret-magic`.

But I'm not going to bother for now.

Test Plan: Ordered by an int field, then reversed the order.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6864

Differential Revision: https://secure.phabricator.com/D14800

src/infrastructure/query/policy/PhabricatorCursorPagedPolicyAwareQuery.php

@@ -724,6 +724,11 @@ abstract class PhabricatorCursorPagedPolicyAwareQuery
           'name' => $field->getFieldName(),
           'aliases' => array($legacy_key),
         );
+
+        $orders['-'.$modern_key] = array(
+          'vector' => array('-'.$modern_key, '-id'),
+          'name' => pht('%s (Reversed)', $field->getFieldName()),
+        );
       }
     }