feat(app/metrics): add extras_js support (#798)

source/app/web/settings.example.json

@@ -28,6 +28,7 @@
   "extras": {
     "default": false,              "//": "Default extras state (advised to let 'false' unless in debug mode)",
     "css": false,                  "//": "Allow use of 'extras.css' option",
+    "js": false,                   "//": "Allow use of 'extras.js' option",
     "features": false,             "//": "Enable extra features (advised to let 'false' on web instances)"
   },
   "plugins.default": false,        "//": "Default plugin state (advised to let 'false' unless in debug mode)",