comments: allow swapping votes.

Allow users to upvote a comment, then downvote it, without raising an
error or having to manually remove the upvote first. The upvote is
automatically removed and replaced by the downvote.

Changes to the /comment_votes API:

* `POST /comment_votes` and `DELETE /comment_votes` now return a comment
  vote instead of a comment.
* The `score` param in `POST /comment_votes` is now 1 or -1, not
  `up` or `down.`

app/models/comment_vote.rb

@@ -1,13 +1,15 @@
 class CommentVote < ApplicationRecord
-  class Error < StandardError; end
-
   belongs_to :comment
   belongs_to :user
+
   validates_presence_of :score
   validates_uniqueness_of :user_id, :scope => :comment_id, :message => "have already voted for this comment"
   validate :validate_comment_can_be_down_voted
   validates_inclusion_of :score, :in => [-1, 1], :message => "must be 1 or -1"
 
+  after_create :update_score_after_create
+  after_destroy :update_score_after_destroy
+
   def self.visible(user)
     if user.is_moderator?
       all
@@ -37,6 +39,18 @@ class CommentVote < ApplicationRecord
     score == -1
   end
 
+  def update_score_after_create
+    comment.with_lock do
+      comment.update_columns(score: comment.score + score)
+    end
+  end
+
+  def update_score_after_destroy
+    comment.with_lock do
+      comment.update_columns(score: comment.score - score)
+    end
+  end
+
   def self.available_includes
     [:comment, :user]
   end