Do all scroll watching on the main window

This moves all scrolling from child divs to the main window. This
improves our keyboard handling, as focus will be given to the
body element, not the child div when navigating by keyboard
commands, so arrow keys and space bar will work after navigating
to the reader with the keyboard commands.

packages/appreader/src/index.jsx

@@ -36,7 +36,6 @@ const App = () => {
           <ArticleContainer
             article={window.omnivoreArticle}
             labels={window.omnivoreArticle.labels}
-            scrollElementRef={React.createRef()}
             isAppleAppEmbed={true}
             highlightBarDisabled={true}
             highlightsBaseURL="https://example.com"