A follow on from
MDL-52969 but across the board for any type of notification email, which isn't locked.
1) Many email clients only support mailto unsubscribe, not urls, so implement notification level unsubscribes via a incoming mail handler
2) If the header is already there (eg thread unsubscribe vs mod_forum notices unsubscribe) then don't add a second one
3) Also add the url across the board and point it to the notifications preference page and also implement the newer One-Click rfc