Custom app install prompt

[corbindavenport]

Link Cleaner should override the 'Add to home screen' prompt on Chrome (and other install banners) with a button that explains installing Link Cleaner will add the app to your device's share menu.

Ideally, the override should only happen if both installation and Web Share Target support are present in the browser. Not sure if there's a way to detect Share Target support with JS, since that's defined in the manifest.