Make it so that when a user removes their own vote, the vote is soft
deleted (the is_deleted flag is set) instead of hard deleted.
Changes:
* Add is_deleted flag to comment votes.
* Relax uniqueness constraint so you can have multiple deleted votes on
the same comment. You can still only have one active vote on the comment.
* Add `soft_delete` method to Deletable concern.
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.`
