[Web] Fix require_once to always include document root

[Web] Add system mails (send mails to all mailboxes via LMTP)
[Web] Allow to add more administrators
[Web] Fix domain administrator editing
[Web] Remove some foreign keys
[Web] Remove username from API
[Web] Remove more .php extension from code
[Web] More minor fixes

data/web/mobileconfig.php

@@ -1,5 +1,5 @@
 <?php
-require_once 'inc/prerequisites.inc.php';
+require_once $_SERVER['DOCUMENT_ROOT'] . '/inc/prerequisites.inc.php';
 
 if (empty($mailcow_hostname)) {
   exit();