Share user sessions

[HansKallekleiv]

Currently we are persisting user selections in sessionStorage in the client browser.
We should investigate if we can persist settings across sessions / users / machines.

One (untested) idea is to have a button that retrieves the current sessionStorage,stores it on the server with a unique hash, and produces an url e.g. my-webviz-app.com#unique-hash that the user can copy.

Using dcc.Location we can listen for such hashes, retrieve the previously stored settings and update the sessionStorage.