Don't manually attempt to close websocket from frontend

There's no need. It gets closed automatically when leaving the page anyway. The reason we haven't been seeing it is that Streamly isn't propagating monadic exceptions properly, which I think is essentially down to composewell/streamly#1203. If we remove the `serverUpdates` stream at line 317, we do see the error handled, and `onDroppedConnection` firing.

js/main.js

@@ -7,8 +7,6 @@ const ws = new WebSocket(wsAddress)
 
 const layouts = JSON.parse(attr("layouts"))
 
-window.onbeforeunload = () => ws.close()
-
 ws.onopen = _event => {
     // Elm
     elmInitialised = false