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/structures/TopLeftMenuButton.js

@@ -89,7 +89,7 @@ export default class TopLeftMenuButton extends React.Component {
                     resizeMethod="crop"
                 />
                 { nameElement }
-                <img className="mx_TopLeftMenuButton_chevron" src="img/topleft-chevron.svg" width="11" height="6" />
+                <img className="mx_TopLeftMenuButton_chevron" src={require("../../../res/img/topleft-chevron.svg")} width="11" height="6" />
             </AccessibleButton>
         );
     }