Make details view content scrollable

Make it possible to scroll the details view if the fields use
more than one vertical screen's worth of space.
Only show field controls for the focused field

When an item field loses focus, hide the Reveal and Copy
buttons.
Load react-style CSS styles in the Chrome extension

react-style injects the compiled CSS into a <style>
element in the document.

In the Chrome extension the app is rendered into a different
document (belonging to the popup view)
than the one which the app code runs in (belonging to the background
page), so we need to copy the <style> element from the background
page document to the popup view's document.