forum: make status labels into clickable filters.

* Make it so that you can click the stickied / locked / deleted icons or
  the new / approved / pending / rejected labels to filter topics by
  that status.

* Replace the `mod_only` search param with `is_private`.

app/views/forum_topics/index.html.erb

@@ -10,8 +10,9 @@
     <p>
       Categories:
       <%= link_to "All", forum_topics_path %>, 
+      <%= link_to "New", forum_topics_path(search: { is_read: false }) %>,
       <% if CurrentUser.is_moderator? %>
-        <%= link_to "Mod+", forum_topics_path(:search => {:mod_only => true}) %>,
+        <%= link_to "Private", forum_topics_path(search: { is_private: true }) %>,
       <% end %>
       <%= ForumTopic::CATEGORIES.map {|id, name| link_to_unless_current(name, forum_topics_path(:search => {:category_id => id}))}.join(", ").html_safe %>
     </p>