Allow sharing of links to a given instance.

We currently can't share a link to the demo configured for a specific instance. It might be possible to make the urls look like "https://mydemoserver.com/urlencodedserverurl/EntityType?filter={...}". From what I remember it should be possible to do so by providing a custom 'history' object (but we should make sure that there is no better way now that react-admin has released a new version).