User request from https://community.metabrainz.org/t/what-do-you-want-in-listenbrainz/444888/165

It would be nice to have an embeddable chunk of HTML to show a user's current playing-now listen on other websites.

It should be a self sufficient bit of HTML code that loads the minimal JS and CSS  required to show the information we need.

Consider implementing fallbacks if there is no currently playing listen: show current pin or last played track instead.

We will want similar embeddable HTML that users can easily copy for our stats graphics: https://art.listenbrainz.org/art/