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