MDL-62889 message_popup: redirect using moodle_url

Without this relative URLs passed (like /message/index.php)
were being redirected to HOST/message/index.php rather than
$CFG->wwwroot/message/index.php.

message/output/popup/mark_notification_read.php

@@ -49,4 +49,4 @@ if ($notification) {
     }
 }
 
-redirect($redirecturl);
+redirect(new moodle_url($redirecturl));