DEV: allows to decorate username selector (PR #9869)

Usage:

api.addUsernameSelectorDecorator(username => {
  return iconHTML("calendar-alt");
});

GitHub

@eviltrout what do you think of this approach? It will allow me to cleanly add an icon like this for example:

Screenshot 2020-05-25 at 15 22 54

Seems good to me!