Fix iPhone homescreen save icon (#379)

* This fixes the icon when saving to homescreen on an iPhone.

* Forgot to change file extension

Author: asvartsjo

web/index.html

@@ -9,6 +9,7 @@
     <meta http-equiv="Expires" content="0" />
     <link rel="manifest" href="/app.webmanifest" />
     <link rel="shortcut icon" type="image/svg+xml" href="/gm.svg" />
+    <link rel="apple-touch-icon" href="/gm.png" />
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
     <link