`us-overlay--open` doesn't do enough to prevent document scrolling.

[greenie]

When using -webkit-overflow-scrolling: touch; inside an open overlay, when you reach the top/bottom of the overlay, the body will continue to scroll.

If we change the implementation in overlay.js to add the class to html instead and update the Sass to:

.us-overlay--active {
  &,
  & body {
    width: 100%;
    height: 100%;
    overflow: hidden;
  }
}

This will fix overlays and allow people to use elastic scrolling within overlays.