This improves both scaling and fairness in the case where a single
track is being sent to large numbers of receivers.  We avoid
communication overhead by growing the number of threads as the
square root of the number of receivers.

Avoids a race.

Kick can no longer be called with groups locked.

This gives the user a friendly message when the server shuts down.

Failed media must still be usable.

We used to display the help message too early, which caused us to
display it even if the group doesn't exist or the user doesn't have
the present permission.

Thanks to Ralf Treinen for pointing out the issue.

Apparently Safari requires {video: true}.

Renegotiation is not correct when the stream is fresh, for example
after the user has switched the camera.

Otherwise the callbacks don't get set.

Closing the channel could cause a panic if a c.write was pending.
Instead, ensure we send a closeMessage to the writer, and cause the
writer to exit when it receives a closeMessage.

We used to silently fail when there was a problem with camera
permissions.  Display a notification.

Thanks to Ines Klimann.