nixos/mailman: don't reserve a static uid in the system

Any system uid will do, so we let the system allocate
one for us. The 'mailman' group is gone entirely since
we don't need it. Users who wish to run the 'mailman'
administration utility can do so via 'sudo':

    $ sudo -u mailman mailman info

Also, simplify the syntax of our user.users entry to
rely on an attribute set rather than a list.