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.`

test/functional/moderator/dashboards_controller_test.rb

@@ -86,9 +86,7 @@ module Moderator
             end
 
             @users.each do |user|
-              CurrentUser.as(user) do
-                @comment.vote!(-1)
-              end
+              create(:comment_vote, score: -1, comment: @comment, user: user)
             end
           end