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/room_settings/ColorSettings.js

@@ -131,7 +131,7 @@ module.exports = React.createClass({
                     if (i === this.state.index) {
                         selected = (
                             <div className="mx_RoomSettings_roomColor_selected">
-                                <img src="img/tick.svg" width="17" height="14" alt="./" />
+                                <img src={require("../../../../res/img/tick.svg")} width="17" height="14" alt="./" />
                             </div>
                         );
                     }