Merge the four media buttons into one.

 * Lots of red.
 * Removes the type_url_form_\$type filter, where \$type is audio, video, file, or image.
 * Replacement filter is a generic type_url_form_media filter for the new unified form.
 * Some functions have gone missing; they'll be restored and deprecated in a later pass.
see #17578.



git-svn-id: https://core.svn.wordpress.org/trunk@18831 1a063a9b-81f0-0310-95a4-ce76da25c4cd