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/PasswordNagBar.js

@@ -31,7 +31,7 @@ export default React.createClass({
         return (
             <div className={toolbarClasses} onClick={this.onUpdateClicked}>
                 <img className="mx_MatrixToolbar_warning"
-                    src="img/warning.svg"
+                    src={require("../../../../res/img/warning.svg")}
                     width="24"
                     height="23"
                     alt=""