Add core support for Playlists and Video Playlists.

* Playlists operate like galleries in the admin. 
* Provide default UI and JS support in themes using MediaElement and Backbone. 
* The shortcodes are clickable, editable, and configurable using the media modal. 
* Playlists support images for each item, whether or not the current theme supports images for `attachment:audio` and `attachment:video`
* Playlists respond to `$content_width` and resize videos accordingly.
* All playlist data is included inline, using a script tag with `type="application/json"`, allowing anyone to unenqueue the WP playlist JS and roll their own.
* Playlist styles are minimal and work out of the box in the last 5 default themes. They inherit and adapt to the current theme's font styles, and their rules are easily overrideable.

See #26631.


Built from https://develop.svn.wordpress.org/trunk@27239


git-svn-id: https://core.svn.wordpress.org/trunk@27096 1a063a9b-81f0-0310-95a4-ce76da25c4cd