Feature: Provide an administrative option to make unsubscribe affect all matching email addresses

Our organization would like the unsubscribe action to unsubscribe all contacts with the same email from the group. In cases of duplicate contacts they unsubscribe once but one of the duplicates will still get an email and then we get CASL complaints. Due to CASL we are trying to be cautious by default here.

We suggest an administrative setting for Civimail: "Unsubscribe contact (radio) or unsubscribe email address (radio)"

There was some support for this option expressed in a discussion here: https://chat.civicrm.org/civicrm/pl/sxd4jpwxpbynx8y15994ua7jea

A good point is that this needs to be guarded by token access to stop attackers being able to unsubscribe prominent emails from organisational lists (for example).

A further suggestion was made to make this a user selection option on the unsubscribe page. I feel that this would be a good but separate feature request.