Permalinks for the playground

It would be nice to give away URLs for the playground that automatically fill in all the parameters in the UI. It could be as simple as adding GET parameters to the URL, and have the javascript code interpret it. Or we could have the server encode/decode the URL, just like it does to generate dynamic context.jsonld (see the blueprints).

We should also add a share button to generate this URLs, or update the location automatically.