Add query string to the start_url to track how often app is launched

Created by: coliff

Useful suggestion from Google in the Web App Manifest documentation. https://developers.google.com/web/fundamentals/web-app-manifest/#start-url

site/docs/4.1/assets/img/favicons/manifest.json

@@ -15,7 +15,7 @@
       "type": "image/png"
     }
   ],
-  "start_url": "/",
+  "start_url": "/?utm_source=a2hs",
   "theme_color": "#563d7c",
   "background_color": "#563d7c",
   "display": "standalone"