fix(console): a VNC console stays open while its screen does not change

nginx ends a proxied websocket that sends nothing for 60 seconds, and the
VNC console sends nothing while the VM's screen stays the same: an idle
console, an installer's progress screen, closed after a minute and had to
be connected again. novncd now pings the browser every 30 seconds. The SSH
tunnel of an SSH compute sends a keepalive as often, for a firewall on the
way that ends idle connections.

console/novncd

@@ -165,4 +165,7 @@ if __name__ == "__main__":
         web=False,
         target_host="ignore",
         target_port="ignore",
+        # a ping while the VM's screen stays the same: nginx, and a proxy on
+        # the way, end a websocket that sends nothing for 60 seconds
+        heartbeat=30,
     ).start_server()