comments: show "undo vote" link by default on voted comments (fix #4143).

app/models/comment.rb

@@ -167,14 +167,14 @@ class Comment < ApplicationRecord
     end
   end
 
-  def below_threshold?(user = CurrentUser.user)
-    score < user.comment_threshold
-  end
-
   def editable_by?(user)
     creator_id == user.id || user.is_moderator?
   end
 
+  def voted_by?(user)
+    user.id.in?(votes.map(&:user_id))
+  end
+
   def hidden_attributes
     super + [:body_index]
   end