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/edit.html.erb

@@ -2,13 +2,7 @@
   <div id="a-edit">
     <h1>Edit Comment</h1>
 
-    <%= error_messages_for "comment" %>
-
-    <%= simple_form_for(@comment) do |f| %>
-      <%= dtext_field "comment", "body" %>
-      <%= f.button :submit, "Submit" %>
-      <%= dtext_preview_button "comment", "body" %>
-    <% end %>
+    <%= render "comments/form", :post => @comment.post, :comment => @comment %>
   </div>
 </div>