Add "mark topics read" link to "View unread posts"

It strikes me that a user ought to have a link in the list of unreads that allows him to mark the all unreads read.

f that were added, I would also suggest that the new link should pass a time to markread() similar to the approach described in http://www.phpbb.com/bugs/phpbb3/52885 so that a user doesn't inadvertently mark as read topics that he never saw because they were posted to after he opened the list of unreads.