Strip down admin-ajax.php to be a bare ajax handler. Move core ajax handlers...

Strip down admin-ajax.php to be a bare ajax handler. Move core ajax handlers to admin/includes/ajax-actions.php. props kurtpayne, ryan. see #15327. see [19738] for initial commit.

git-svn-id: https://core.svn.wordpress.org/trunk@19739 1a063a9b-81f0-0310-95a4-ce76da25c4cd