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/globals/NewVersionBar.js

@@ -96,7 +96,7 @@ export default React.createClass({
         }
         return (
             <div className="mx_MatrixToolbar">
-                <img className="mx_MatrixToolbar_warning" src="img/warning.svg" width="24" height="23" />
+                <img className="mx_MatrixToolbar_warning" src={require("../../../../res/img/warning.svg")} width="24" height="23" />
                 <div className="mx_MatrixToolbar_content">
                     {_t("A new version of Riot is available.")}
                 </div>