CiviMail: add information about the mailing that triggered an unsubscribe

As per http://forum.civicrm.org/?topic=6428#msg28368 - we only show that a contact removed itself from a group through unsubscribing, but don't show what mailing prompted them to do this. That information is stored in civicrm_mailing_event_unsubscribe.

When displaying that a contact removed themselves from a group through unsubscribing, add a link to the mailing that caused them to do this.