Add MTA for Mailman and Hydra

This is useful for sending build failure notifications and for serving public malinglists in conjunction with Mailman.

Things left to do in no particular order:

[ ] Rewrite the type for config so that whenever something like config.alias_maps.hash is specified, the corresponding map file is automatically created.
[ ] Run each Postfix service as a different user.
[ ] Provide a sane default configuration.
[ ] Add module assertions for impossible service configuration combinations.
[ ] Improve NixOS VM test.
[ ] Handle processLimit.
[ ] Set ambient capabilities for Postfix services.
[ ] Move sockets to /run.
[ ] Implement chroot and namespacing.
[ ] Add a postfix.target to better control all Postfix service units at once.
[ ] Implement waking up Postfix services.
[ ] Strip down postmap to just map file creation.
[ ] Properly set permissions on sockt files.
[ ] Get rid of postdrop group.
[ ] Add an option for Postfix service description.
[ ] Implement custom merge function for the config option so that scalars is merged using lists.
[ ] Take care of helper programs like sendmail or postqueue.
[ ] Perform extra validation of the config file during build time.
[ ] Log to stderr instead of syslog.
[ ] Adjust throttling of service restarts.
[ ] Implement high level abstractions (like eg. cfg.maps or cfg.restrictions).

headcounter / deployment