Unify mutableUsers = { true, false }

With mutableUsers = true, we now ensure that all users and groups that
were created declaratively, are updated or removed
appropriately. Thus, adding a user to users.extraUsers and then
removing it now causes the acoount to be removed from
/etc/passwd. Thus user/group management is fully congruent except that
users and groups that were created imperatively (via useradd/groupadd)
are not touched. We distinguish between declarative and imperative
users/groups by tracking the former in
/var/lib/nixos/declarative-{groups,users}.

With mutableUsers = false, you are now no longer required to specify
UIDs/GIDs for all users. The handling of mutableUsers = true/false is
the same code path; the only difference is that the "false" mode
ignores the existing contents of /etc/{passwd,group}.

The attribute ‘createUser’ is gone. It doesn't really make sense to
specify users that shouldn't be created.