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)

---

Migrated lists:

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

roles/mailman3/templates/settings.py.j2

@@ -46,3 +46,5 @@ EMAIL_PORT = 25
 #: Sender in Emails sent out by Postorius.
 DEFAULT_FROM_EMAIL = 'postorius@{{ lists_domain }}'
 SERVER_EMAIL = 'root@{{ lists_domain }}'
+
+POSTORIUS_TEMPLATE_BASE_URL = 'http://localhost'