For consistency with the rest of the UI elements rendered by the
annotator code, move the warning banner at the top of the PDF into
shadow DOM. This is also another step towards not needing to load any
global CSS (eg. our CSS reset) into the host page.

 - Change the container element for the banner from `<div>` to
   `<hypothesis-banner>` to match other `<hypothesis-*>` UI container
   elements
 - Move the `WarningBanner` component into `components/WarningBanner.js`
   and rename the CSS classes to match