Add meta tag for official iOS app (#16599)

app/views/layouts/application.html.haml

@@ -23,6 +23,7 @@
     %link{ rel: 'manifest', href: manifest_path(format: :json) }/
     %meta{ name: 'theme-color', content: '#6364FF' }/
     %meta{ name: 'apple-mobile-web-app-capable', content: 'yes' }/
+    %meta{ name: 'apple-itunes-app', content: 'app-id=1571998974' }/
 
     %title= content_for?(:page_title) ? safe_join([yield(:page_title).chomp.html_safe, title], ' - ') : title