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.`
* Add comment scores.
* Rework voting buttons so that you can click the upvote/downvote
buttons to toggle votes.
* Hide the edit, delete, undelete, and report buttons behind a popup menu.
* Show the upvote/downvote/reply buttons to logged out users. Redirect
them to the login page instead.
