Merge branch 'optional_outgoing_bcc' into 'stable-3.1'

sender_cc must be optional

See merge request ispconfig/ispconfig3!1083

Author: Till Brehm

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

@@ -199,7 +199,8 @@
 				2 => array( 'event' => 'SAVE',
 					'type' => 'TOLOWER')
 			),
-			'validators' => array (  0 => array ( 'type' => 'ISEMAIL',
+			'validators' => array (  0 => array ( 'type' => 'ISEMAILADDRESS',
+					'allowempty' => 'y',
 					'errmsg'=> 'sender_cc_error_isemail'),
 			),
 			'default' => '',