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/RoomDirectory.js

@@ -571,7 +571,7 @@ module.exports = React.createClass({
         const DirectorySearchBox = sdk.getComponent('elements.DirectorySearchBox');
         return (
             <div className="mx_RoomDirectory">
-                <SimpleRoomHeader title={ _t('Directory') } icon="img/icons-directory.svg" />
+                <SimpleRoomHeader title={ _t('Directory') } icon={require("../../../res/img/icons-directory.svg")} />
                 <div className="mx_RoomDirectory_list">
                     <div className="mx_RoomDirectory_listheader">
                         <DirectorySearchBox