add `address` argument to unsubscribe command
Complementary of #721 (closed)
For GDPR compliance the unsubscription should be as easy as subscription. Due to the subscription form it is possible to subscribe a plus-address by confirming with the main address (e.g. if I enter name+mailman@example.com I can confirm the request that gets send to name@example.com). Unsubscribing via mail command is not possible (as I may only be able to send with name@example.com) and there is no button to unsubscribe in mailman 3. So to be able to unsubscribe name+mailman@example.com I first have to create an account with that address and only then am I able to unsubscribe.
Some providers may allow users to send from their plus-address but not all. This may also affect users of masked email services (i.e. Firefox Relay or addy.io) where it might not be possible to send a mail from the subscribed mail address.