Confirm email address to be unsubscribed

As a user, I want to check the email address being unsubscribed to not unsubscribe someone else's email address when an email was forwarded to me.

This issue arose from issue 15251 (requires access to security issues).

Our concern is that when emails are forwarded that someone may hit the unsubscribe link under the belief that they will unsubscribe from the forwarder's emails, but actually they are unsubscribing the forwarder from the CiviCRM install.