Add cache-first service worker

client/src/js/boot.js

@@ -0,0 +1,14 @@
+/* eslint-disable no-underscore-dangle */
+
+// This entrypoint gets inlined in the index page cached by service workers
+// and is responsible for fetching the data we would otherwise embed
+
+window.__env__ = fetch('/data', {
+  credentials: 'same-origin'
+}).then(res => {
+  if (res.ok) {
+    return res.json();
+  }
+
+  throw new Error(res.statusText);
+});