Setup mailman3 server

We want to migrate to mailman3 as mailman2 is basically unmaintained and
requires Python 2 which is EOL.

Because the mailman and mailman3 packages conflict and we don't want to
perform a big bang migration, mailman3 must be deployed on a separate
server. mailman-web (mailman3's web interface) hasn't been packaged yet,
so for now we are using my homebrewed PKGBUILD[1].

[1] https://gist.github.com/klausenbusk/5982063f95c503754a51ed2fefb8915e

Ref #59