Allow filtering attachments by Author name in Media Library. props greuben. fixes #16044.

git-svn-id: http://core.svn.wordpress.org/trunk@23430 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Replace get_user_id_from_string() with get_user_by(). props ocean90, bananastalktome. fixes #23192. see #23190.

git-svn-id: http://core.svn.wordpress.org/trunk@23431 1a063a9b-81f0-0310-95a4-ce76da25c4cd