Discourse now works as a PWA in iOS

[Falco]

It is available on Android for over four years now.

Fully support Android Chrome's "Add to Homescreen"

By using @tgxworld work, I just added more entries to get a better app feeling. This PR makes the window standalone (no address bar), adds the header background color to the Android top bar and uses the Discourse background color on the splash screen background so the transition is smoother. https://github.com/discourse/discourse/pull/4099 I made a small video too: