Adds option to duplicate articles.

Requires that you have access to edit the original article,
resets publishing and expiration date. New article's ID is
added to the new Title and the URL.

Closes issue #396

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5583 2fea6a4d-a838-0410-917b-93a53c48e9d2