LDAP user account expiry and deletion

The administration of users in Mahara that is connected to a LDAP directory should be simplified to avoid high administrative burden. That means when users are deleted from LDAP, the next time cron runs on Mahara, the accounts that cannot be found anymore are marked as “expired” automatically. Administrators can then view all expired accounts at /admin/users/suspended.php by choosing the option “Expired users” from the drop-down menu. Account expiry will only be done when there is a successful connection to LDAP to avoid accidental expiry of all accounts due to a connection error.