User Lookup: Add Account Username

This commit adds user's account username as one of the field to scan on quick lookup;

include/ajax.users.php

@@ -77,6 +77,7 @@ class UsersAjaxAPI extends AjaxController {
                     'emails__address__contains' => $q,
                     'name__contains' => $q,
                     'org__name__contains' => $q,
+                    'account__username__contains' => $q,
                 ));
                 if (UserForm::getInstance()->getField('phone'))
                     $filter->add(array('cdata__phone__contains' => $q));