comments.js: fix thresholded comments.

* Restore behavior of thresholded comments being greyed out (lost in 6fa0ae2cf).

* Set the `below-threshold` class for thresholded comments in the html instead of in javascript.

* Remove `include_below_threshold` param; it was always true when clicking "Show all comments".

app/controllers/comments_controller.rb

@@ -70,7 +70,6 @@ private
   def index_for_post
     @post = Post.find(params[:post_id])
     @comments = @post.comments
-    @comments = @comments.visible(CurrentUser.user) unless params[:include_below_threshold]
     render :action => "index_for_post"
   end