Allow admins to remove comment votes by other users. This is done by
clicking the comment score to get to the comment vote list, then
clicking the Remove button on every vote.
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.`
