Merge branch 'patch-search-on-mailbox-login' into 'develop'

Allow searching on the mailbox login field

See merge request ispconfig/ispconfig3!1340

Author: Marius Burkard

interface/web/mail/form/mail_user.tform.php

@@ -122,7 +122,8 @@
 			'default' => '',
 			'value'   => '',
 			'width'   => '30',
-			'maxlength' => '255'
+			'maxlength' => '255',
+			'searchable' => 2
 		),
 		'password' => array (
 			'datatype' => 'VARCHAR',