Remove global css from DOM when Webamp is unmouted

Generally, we need to find a better way to include css.

* It’s the main blocker still stopping us from dropping Webpack from the NPM build, and thus the only thing stopping us from shipping a non-bundled version. This would let people reuse the version of React which is already on their page.
* CSS should be removed from the DOM when the component is unmonted.
* If there are multiple versions of Webamp on a page, we should only add on set of global CSS to the DOM. Maybe some kind of global variable that holds a reference count?