CiviMail - Verify / Fix Unsubscribe Functionality

For 1.6, let's review and fix the existing specified unsubscribe functionality. (Subsequently we can look at re-architecting the relationship between Groups and Mailings....)

Here's how things should be working...

• Unsubscribe should set the contact's civicrm_group_contact.status to "Removed" for EACH GROUP that is a target of the mailing they are unsubscribing from. This applies to both static and smart groups. The code should check if a group_contact record exists for that group_id + contact_id. If exists, update status to "Removed", if not then INSERT a group_contact record with status = Removed. Note that this will always result in an INSERT for mailing recipients where they got the mailing due to being a "smart group" member. It will also result in INSERTs of a removed status for the case where the mailing was sent to 2+ static groups and contact was only a member of one (they will wind up being "Removed" from both). Lobo and I discussed this and feel that this simpler broad-brush approach is the best for now.

• opt-out should set the civicrm_contact.is_opt_out to TRUE (1)

• subsequent mailings to a GROUP (smart or static) should exclude
  contacts with status="Removed"

• all mailing should exclude contacts with is_opt_out = TRUE