Add websocket support to export feature on the client side

[bmblb]

Currently, when trying to use export server runnin on websocket with default settings, you run into issue, reproducible in chrome:

1. run export server for websocket: node src/server.js -w 8080
2. run export demo, try to export

OPTIONS http://localhost:8081/ 404 (Not Found)
Access to fetch at 'http://localhost:8081/' from origin 'http://localhost:8080' has been 
blocked by CORS policy: Response to preflight request doesn't pass access control check: 
It does not have HTTP ok status.
Uncaught (in promise) TypeError: Failed to fetch

websocket requires special handling:

let ws = new WebSocket(url);
ws.onopen = function() { ws.send(request); }
ws.onmessage = function () { ws.close(); }