Update to GNU mailman 3

[aninrusimha]

GNU Mailman 2.1 requires Username / password SASL auth if you wish to use TLS auth
I have hotfixed this issue by commenting out code in /var/lib/mailman/Mailman/Handlers/SMTPDirect.py,
but we really need the more nuanced configuration options that were introduced in 3.0

Docs for mailman 3:
https://docs.mailman3.org/projects/mailman/en/latest/README.html
Converting:
https://docs.mailman3.org/en/latest/migration.html

[robertquitt]

Additionally, mailman 3 has an API for managing list members instead of only via the command line

[aninrusimha]

Installed on tap to /etc/mailman3 and imported the lists from mailman2, will try to transition later today