Until now we always displayed the same feature images on the landing
page and it didn't matter if the user was on a desktop or mobile device.
This is now changed so that we display different images on mobile
devices (depending on the screen resolution).
Besides that we also moved the deprecated `themeColor` from the
`Metadata` to the `Viewport` and added it to all pages.
Last but not least we added some missing Android icons, which were
referenced in the `manifest.json` file, but where the icon didn't exist.
