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/controllers/comments_controller.rb

@@ -23,12 +23,12 @@ class CommentsController < ApplicationController
   def update
     @comment = Comment.find(params[:id])
     check_privilege(@comment)
-    @comment.update_attributes(params[:comment].permit(:body))
+    @comment.update(update_params, :as => CurrentUser.role)
     respond_with(@comment, :location => post_path(@comment.post_id))
   end
 
   def create
-    @comment = Comment.create(params[:comment])
+    @comment = Comment.create(create_params, :as => CurrentUser.role)
     respond_with(@comment) do |format|
       format.html do
         if @comment.errors.any?
@@ -110,4 +110,12 @@ private
       raise User::PrivilegeError
     end
   end
+
+  def create_params
+    params.require(:comment).permit(:post_id, :body, :do_not_bump_post, :is_sticky)
+  end
+
+  def update_params
+    params.require(:comment).permit(:body, :is_deleted, :is_sticky)
+  end
 end