pundit: convert forum topics / forum posts to pundit.

Fix it being possible for users to delete or undelete their own forum
posts and topics, even if they were deleted by a mod.

app/views/forum_topics/index.html.erb

@@ -11,7 +11,7 @@
       Categories:
       <%= link_to "All", forum_topics_path %>, 
       <%= link_to "New", forum_topics_path(search: { is_read: false }) %>,
-      <% if CurrentUser.is_moderator? %>
+      <% if policy(ForumTopic).moderate? %>
         <%= 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 %>