- Add websocket client to create connection with SockJS websocket server.
It contains callback method to launch after connection setup.
- Add '--websocket' parameter to 'check_send_receive_time' script to
check websocket connection.
- Add testing websocket connection to production installation checking.
- Add cronjob to launch websocket connection nagios test.
This makes it possible for Zulip Nagios monitoring to check for
problems impacting the websockets sending code path, which is what all
web users use.
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