Use an Object.assign polyfill

Summary:
I was running into facebook/react#6451 even though I'm on React 15.3.1
and `npm ls | grep object-assign` shows only `object-assign@4.1.0`. A
bit of debugging shows that the polyfill wasn't being applied early
enough in the node bundle that's used for server-side rendering; we
have to be sure to include it before any React components are used, so
we might as well make it the very first thing!

Test Plan:
Load localhost:8080/skills and see that there aren't any SSR errors.

package.json

@@ -38,6 +38,7 @@
         "eslint-plugin-react": "^6.1.2",
         "file-loader": "^0.9.0",
         "normalize.css": "^4.2.0",
+        "object-assign": "^4.1.0",
         "static-site-generator-webpack-plugin": "^2.1.0",
         "webpack": "^1.13.2",
         "webpack-dev-server": "^1.15.0"

src/index.js

@@ -1,12 +1,23 @@
 /*
  * Main entry point. Exports a function to do server-side rendering,
  * and runs client initialization (rehydration) code if on the frontend.
+ *
+ * Note that this module _is_ transpiled by Babel. However, we want to
+ * require the `object-assign` polyfill before _anything_ else happens.
+ * As a consequence, we can't use the ES6 module syntax, because Babel
+ * hosts these imports---including `export ... from` forms---to the very
+ * top of the file. So we manually define the exports, old-school style.
  */
 
-import initializeClient from './client';
+Object.assign = null;
+Object.assign = require("object-assign");
 
-if (typeof document !== "undefined") {
-    initializeClient();
+const isServerRendering = typeof document === "undefined";
+
+if (!isServerRendering) {
+    require("./client").default();
 }
 
-export {default} from './server';
+module.exports = {
+    default: require("./server.js").default,
+};