[FEATURE] Add JS event on Websocket disconnect

[philippjfr]

When a Bokeh server application times out and disconnects from the Websocket there's generally no indication that this happened and indeed no mechanism for a developer to add some kind of message. To enable this it would be great if we could somehow allow declaring a JS callback which fires when the Websocket disconnects which could then be leveraged by a developer to pop up a notification of some kind.

[bryevdv]

Let's try to add this for 3.0

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