Create a new file, `media-audiovideo.js`, to house all of the audio and video JS code in core.

UX Changes:
* Don't add a menu item for "Add Audio|Video Source"
* In the Audio|Video Details modal, add buttons and some suggestive text for adding alternate playback sources
* Don't show "Create Audio|Video Playlist" menu items until the user has uploaded audio or video files

See #27437.



git-svn-id: https://develop.svn.wordpress.org/trunk@27608 602fd350-edb4-49c9-b593-d223f7449a82