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.`
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