Replace check_permissions() with ajax_user_can(). New method returns...

Replace check_permissions() with ajax_user_can(). New method returns true/false to current_user_can(), which we then handle in admin ajax. see #15326.

git-svn-id: https://core.svn.wordpress.org/trunk@16992 1a063a9b-81f0-0310-95a4-ce76da25c4cd