title: Detect if app is installed on iOS device and display appropriate message
body: <p>I want users who access my website via their iOS device an html page that displays a message:
  "App is installed" if a specific app is installed (not any app - but a specific app I'm working on) and "App is not installed" if it is not installed.</p>
  
  <p>I don't want to launch the app - just show the right message.</p>