Turn off/on unread replies subscription for a topic as well as sets individual topic's alert preferences
Must be called with a topic specified in the URL.
The mode can be from 0 to 3
0 => unwatched, 1 => no alerts/emails, 2 => alerts, 3 => emails/alerts
Upon successful completion of action will direct user back to topic.
Accessed via ?action=unwatchtopic.