Add a class property to `wp.media.view.MediaDetails` called `instances` that is incremented every time the class is invoked. This mimics what happens in the shortcodes: the sources receive an incremented cache-buster so that the browser won't ignore the file when `preload="metadata"` is set.

See #26779.



git-svn-id: https://develop.svn.wordpress.org/trunk@27520 602fd350-edb4-49c9-b593-d223f7449a82