Use mandrill unsubscribe tags

[Andy Millne]

Mandrill has the ability to process and track unsubscribe links using the merge tag

http://help.mandrill.com/entries/23815367-Can-I-add-an-automatic-unsubscribe-link-to-Mandrill-emails-

We should consider adding support for this but as mandrill places a block on sending we should check that this will not interfere with non mailing list items.