Simplify creation of audio, video, and playlist MCE views by placing them in iframe sandboxes.

Wins:
* Eliminates duplication of code between PHP and JS
* Views can load JS without messing with TinyMCE and scope
* MEjs doesn't break when it loads a file plugin-mode. This allows any file type the MEjs supports to play in MCE views.
* YouTube now works as the source for video.
* Users can still style the views, editor stylesheets are included in these sandboxes.
* Audio and Video URLs and `[embed]`s are no longer broken.
* Remove the crazy compat code necessary to determine what file types play in what browser.
* Remove unneeded Underscore templates.
* Remove the compat code for playlists.

See #28905.

Built from https://develop.svn.wordpress.org/trunk@29179


git-svn-id: http://core.svn.wordpress.org/trunk@28963 1a063a9b-81f0-0310-95a4-ce76da25c4cd