Use Workbox CDN as default (facebook#5180)

Author: Timer

config/webpack.config.prod.js

@@ -492,7 +492,7 @@ module.exports = {
     new WorkboxWebpackPlugin.GenerateSW({
       clientsClaim: true,
       exclude: [/\.map$/, /asset-manifest\.json$/],
-      importWorkboxFrom: 'local',
+      importWorkboxFrom: 'cdn',
       navigateFallback: publicUrl + '/index.html',
       navigateFallbackBlacklist: [
         // Exclude URLs starting with /_, as they're likely an API call