Add mailman role for the new lists.al.org machine

group_vars/all/vault_mailman.yml

playbooks/lists.archlinux.org.yml

roles/mailman/defaults/main.yml

roles/mailman/files/mailman.ini

roles/mailman/files/milter_header_checks

roles/mailman/handlers/main.yml

roles/mailman/tasks/main.yml

roles/mailman/templates/main.cf.j2

roles/mailman/templates/mm_cfg.py.j2

roles/mailman/templates/nginx.d.conf.j2