Convert image URLs in React to require calls

This allows Webpack to insert the proper image URL after builds steps like
adding a hash and so on. The path you supply to `require` is relative to the JS
source file, just like any other would be.

src/Avatar.js

@@ -56,6 +56,6 @@ module.exports = {
         for (let i = 0; i < s.length; ++i) {
             total += s.charCodeAt(i);
         }
-        return 'img/' + images[total % images.length] + '.png';
+        return require('../res/img/' + images[total % images.length] + '.png');
     },
 };