Implement app clustering (#17752)

* Install throng for easy cluster management
* Extract the Express app construction into its own file
* Switch server.js to use app clustering for deployed environments
* Worker count is based on the lesser of process.env.WEB_CONCURRENCY and the count of CPUs
* Reading clustered output is difficult, let's prefix the std{out,err} streams

Co-authored-by: Jason Etcovitch <jasonetco@github.com>

app.json

@@ -3,7 +3,8 @@
   "env": {
     "NODE_ENV": "production",
     "NPM_CONFIG_PRODUCTION": "true",
-    "ENABLED_LANGUAGES": "en"
+    "ENABLED_LANGUAGES": "en",
+    "WEB_CONCURRENCY": "1"
   },
   "buildpacks": [
     { "url": "heroku/nodejs" }