Remove STANDBY_IMAGE_URL support.

This feature only works when /hls streams are viewed over RTMP
and it causes more trouble than it is worth. I would like this
feature but it will require a different implementation.
Mark sample command inside GFM code block.
Fix /player RTMP application.

`$name` is only available with exec_*. Switch from push and push_exec to
pull from /player with implicit stream name and live=1.