Preview link broken for posts assigned a custom status

Both [core](http://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/meta-boxes.php#L45) and the calendar apply the `preview_post_link` filter to the permalink. 

Edit Flow was using this filter to correct the preview link, but @cojennin removed that in ac462ad for reasons that aren't communicated in the changeset.