Add links to icons for forums and topics with no unread posts

Since v3.2.0 the icons at the left hand side of the index, forum, and search results pages have been clickable, as links to the forum or first unread post, but only if the forum or topic contained unread posts. Icons next to forums or topics with no unread posts are not clickable.

I am proposing a patch that gives icons for forums with no unread posts a link to the forum (same as for forums with unread posts), and icons for topics with unread posts a link to the last post in the topic (which seems to be the natural thing to do as an extension of the behaviour for topics with unread posts).