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/components/views/login/CountryDropdown.js

@@ -70,7 +70,7 @@ export default class CountryDropdown extends React.Component {
     }
 
     _flagImgForIso2(iso2) {
-        return <img src={`img/flags/${iso2}.png`} />;
+        return <img src={require(`../../../../res/img/flags/${iso2}.png`)} />;
     }
 
     _getShortOption(iso2) {