Migrate demo to gulp (#3152)

* Migrate demo to gulp

* Tweak demo migration to gulp

* Feature detect demo

* Gen icons

webpack.config.js

@@ -1,5 +1,7 @@
 const { createAppConfig } = require("./build-scripts/webpack.js");
 
+// This file exists because we haven't migrated the stats script yet
+
 const isProdBuild = process.env.NODE_ENV === "production";
 const isStatsBuild = process.env.STATS === "1";