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

Ref #59 (closed)

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Migrated lists:

• arch-announce
• arch-devops-private
• arch-events
• arch-wiki-admins

roles/nginx/defaults/main.yml

 ---
 letsencrypt_validation_dir: "/var/lib/letsencrypt"
+nginx_firewall_zone: