Most users don't realize that the Featured Image meta box exists; if they do, few use it.
Restores the old meta box UI, including the admin_post_thumbnail_html filter. If a plugin is using _wp_post_thumbnail_html() in conjunction with Thickbox elsewhere, it will also magically still work.
Specific underlying changes:
* Converts the modal view to use the view manager, which means that a call to open() will automatically call render and attach if necessary.
* Doesn't automatically set a state in wp.media, to allow code to customize the states to be added before activation.
props koopersmith.
fixes#21776.
git-svn-id: https://develop.svn.wordpress.org/trunk@22979 602fd350-edb4-49c9-b593-d223f7449a82
