Update the JS tooling

* get rid of gulp
* use webpack-dev-server to serve app.js
* express serves static files
* 2 webpack configs: local and production
* for development with backend, websocket URL is injected
* remove unneeded static files

client/app/scripts/local.js

@@ -0,0 +1,7 @@
+// This file is an entrypoint for development,
+// see main.js for the real entrypoint
+
+// Inject websocket url to dev backend
+window.WS_URL = 'ws://' + location.hostname + ':4040';
+
+require('./main');