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/policies/comment_vote_policy.rb

@@ -1,7 +1,10 @@
 class CommentVotePolicy < ApplicationPolicy
+  def create?
+    unbanned? && !record.comment.is_deleted?
+  end
+
   def destroy?
-    # XXX permissions are checked in Comment#unvote!
-    true
+    record.user_id == user.id
   end
 
   def can_see_votes?