Media modal: fix reordering of the thumbnails while creating or editing a gallery, props avryl, fixes #29606 for trunk. 
Built from https://develop.svn.wordpress.org/trunk@29876


git-svn-id: http://core.svn.wordpress.org/trunk@29633 1a063a9b-81f0-0310-95a4-ce76da25c4cd