Rewrite frontend in React (#1273)

I rewrote the frontend in React using a module bundler. It's matched feature-for-feature with the current frontend, with only slight changes in the styling. I did not fuss about making the styling identical; the badge popup looks particularly different.

This makes the front end much easier to develop. I'm really looking forward to implementing #701, to which this paves the way.

This makes light use of Next.js, which provides webpack config and dev/build tooling. We’ll probably replace it with create-react-app or our own webpack setup because unfortunately it comes with a lot of runtime overhead (the build is 400k).

Let’s open new issues for bugs and features, and track other follow-ups here: https://github.com/badges/shields/projects/1

frontend/components/header.js

@@ -0,0 +1,24 @@
+import React from 'react';
+
+export default () => (
+  <section>
+    <img alt="Shields.io" src="static/logo.svg" />
+
+    <hr className="spacing" />
+
+    <p className="highlights">
+      Pixel-perfect &nbsp;
+      Retina-ready &nbsp;
+      Fast &nbsp;
+      Consistent &nbsp;
+      Hackable &nbsp;
+      No tracking
+    </p>
+
+    <style jsx>{`
+      .highlights {
+        font-style: italic;
+      }
+    `}</style>
+  </section>
+);