media browser should list newest media first

The media browser that pops up when inserting media into a WYSIWYG textarea currently presents the oldest media first. This makes it hard to find new site content. It would make sense to show the newest media first.

From reading other issues, it seems like the media browser has some potential revisions down the road. However, this is a really simple patch that improves basic usability--for developers testing and for actual users.