Add "user" and "users" standard custom fields

Summary: These end up a little weird with subclassing instead of `switch`, but some day we could alias them to one another or something I guess. If I'm feeling brave, I might get rid of the "user" variant when I migrate Maniphest custom field specs, and turn it into "users, limit = 1" or something like that.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7010

src/applications/search/engine/PhabricatorApplicationSearchEngine.php

@@ -504,7 +504,7 @@ abstract class PhabricatorApplicationSearchEngine {
     $handles = array();
     if ($all_phids) {
       $handles = id(new PhabricatorHandleQuery())
-        ->setViewer($this->getViewer())
+        ->setViewer($this->requireViewer())
         ->withPHIDs($all_phids)
         ->execute();
     }