Add a List-Unsubscribe field for mails sent via team subscription

For any mail received through a direct package subscription, you get a List-Unsubscribe header indicating you to send a mail with unsubscribe <package> to control@tracker.debian.org.

We should add the same header for mails sent through a team subscription and it should advise to send a leave-team <team-id> command to control@tracker.debian.org.

Relevant code for this is in distro_tracker/mail/dispatch.py (and tests in distro_tracker/mail/tests/tests_dispatch.py).