Perform a canonical redirect for posts that are published but are visited at a `?p=123&preview=true` URL.

Props amit, joostdevalk.
Fixes #20496.

Built from https://develop.svn.wordpress.org/trunk@28874


git-svn-id: http://core.svn.wordpress.org/trunk@28673 1a063a9b-81f0-0310-95a4-ce76da25c4cd