postfix: sort relay_domains, and fix whitespace
postfix: Move main.yml to postfix.yml
mailman: Use a more consistent name for mailman config option
postfix: Drastic cleanup of postfix role

The postfix role was a mess. This commit cleans up most of the rubbish
therein, and makes it possible to manage a wide range of postfix
configurations using only ansible variables, rather than replicating
each and every postfix config tree in this repository.

Postfix main.cf is now managed as a template in the postfix role,
allowing us to easily roll out security modifications to the role across
the infrastructure.
Remove questionable RT configuration playbook
postfix: Allow extending $mynetworks
s116: add lists.irail.be to relay_domains, remove some cruft
s116: correct typo in relay_domains
s116: add lists.p4od.org domain