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/models/comment.rb

@@ -189,6 +189,10 @@ class Comment < ApplicationRecord
     true
   end
 
+  def below_threshold?(user = CurrentUser.user)
+    score < user.comment_threshold
+  end
+
   def editable_by?(user)
     creator_id == user.id || user.is_moderator?
   end