Add 'post as moderator' option for comments.

* Add 'post as moderator' option to comment form. This creates a so-called sticky comment.
* Downvotes have no effect on stickied comments; they're always visible, regardless of comment thresholds.
* Only mods may sticky comments.
* Mods may sticky comments by other users.

app/views/comments/partials/index/_list.html.erb

@@ -28,7 +28,7 @@
   <% if CurrentUser.is_member? %>
     <div class="new-comment">
       <p><%= link_to "Post comment", new_comment_path, :class => "expand-comment-response" %></p>
-      <%= render "comments/partials/new/form", :post => post %>
+      <%= render "comments/form", :post => post, :comment => post.comments.new %>
     </div>
   <% end %>
 </div>