Change post votes to work the same way as comment votes:
* Make the upvote arrow blue if you've upvoted the post, or grey if you
haven't. Likewise for the downvote arrow.
* Make it so you can click the upvote or downvote arrows to undo the vote.
* Don't show any notices when you vote on a post.
Also fix it so that votes work the same way on the posts page, the
comments page, and in the modqueue. Before it wasn't possible to undo
votes on the comments page or in the modqueue.
* Clean up javascript.
* Return HTTP 422 instead of HTTP 500 on "you have already voted for
this post" errors.
* In json/xml error responses, return the error message in the `message`
field, not `reason`.
* In json/xml success responses, return the post itself instead of a
plain `{ success: true }` object.
