Is called to turn off/on notification for a particular topic.

• Must be called with a topic specified in the URL.
• Uses the Notify template (main sub template) when called with no subaction.
• The sub action can be 'on', 'off', or nothing for what to do.
• Requires the mark_any_notify permission.
• Upon successful completion of action will direct user back to topic.
• Is accessed via ?action=notify.