Make it possible for commenter cookies to be disabled if someone wants to by...

Make it possible for commenter cookies to be disabled if someone wants to by setting them on an action instead of always. Fixes #17976 props SergeyBiryukov and pishmishy .


git-svn-id: https://core.svn.wordpress.org/trunk@19622 1a063a9b-81f0-0310-95a4-ce76da25c4cd