Minor update for nginx site conf
Fix issue with unkickable attendee

When attendee is disconnected due to flaky network, gevent-socketio is
unable to detect attendee being disconnected (even though they claim
they do.)  So when kick event is triggered, attendee will not be able to
response due to disconnection. Hence causing the attendee to be
unkickable.

Until the root cause of gevent-socketio unable to detect attendee
disconnection is found, we will first keep track of attendees who are
unkickable. Then when they are kicked again, we manually force
disconnect their associated sockets, which effectively kick them out of
the conference room.