Only render the main component with a page

Do not render the main component when there is no page yet. So we mount
the main component only on the first `App.render` call. That’s the
point that where we actually want to render a page component.
Make the navigation component functional

Include its text in the locale files. And add an active item indicator.
This indicator is passed down via the main component based on the page’s
key property. This key must match the locale key of the menu item.