cmd, dashboard: use webpack dev server, remove custom assets (#16263)

* cmd, dashboard: remove custom assets, webpack dev server

* dashboard: yarn commands, small fixes

dashboard/assets/webpack.config.js

@@ -32,6 +32,9 @@ module.exports = {
 			mangle:   false,
 			beautify: true,
 		}),
+		new webpack.DefinePlugin({
+			PROD: process.env.NODE_ENV === 'production',
+		}),
 	],
 	module: {
 		rules: [