implement conference challenge

[kra]

4 achievements
each is the prerequisite for the next
conference-2 conference-4 conference-8 conference-16

whenever the conference has a population of N or greater, conference-N is awarded to all conference participants
we would have to poll every 30 seconds or something, can we even get the conference participants?
can use the same supervisord script as for the client-status-* heartbeat poller