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/mailman3/templates/settings.py.j2

-# Mailman Web configuration file.
-# /etc/mailman3/settings.py
+# mailman-web config
 
 from mailman_web.settings.base import *
 from mailman_web.settings.mailman import *
@@ -31,7 +30,6 @@ ALLOWED_HOSTS = [
 #: site, see: https://docs.djangoproject.com/en/dev/ref/contrib/sites/
 SITE_ID = 1
 
-
 SECRET_KEY = '{{ vault_mailman_web_secret_key }}'
 
 MAILMAN_REST_API_USER = '{{ vault_mailman_admin_user }}'