Re-Add QR-code for installing mobile app

[phavekes]

The QR code was removed from the homepage in #852

Re-add a QR-code:

• Pointing to a internal url /install-app
• On this page, the mobile OS is detected
• The user is sent to the Android or IOS url
• If OS detection fails, the user is redirected to the homepage
• the qr code is hidden in mobile view

Example OS detection:

function getMobileOperatingSystem() {
    var userAgent = navigator.userAgent || navigator.vendor || window.opera;

    if (/android/i.test(userAgent)) {
        return "Android";
    }

    if (/iPad|iPhone|iPod/.test(userAgent) && !window.MSStream) {
        return "iOS";
    }

    return "unknown";
}