Only add Mailman related config to the Postfix config if Mailman is installed

Currently, in all Postfix configurations, there are references to Mailman, e.g. for the transport. We should only put that in there if Mailman is installed. This will prevent issues such as https://www.howtoforge.com/community/threads/after-migrate-from-debian-10-to-11-i-cant-use-an-email-that-existing-in-mailing-list.88681/