Like 9efb374ae, allow users to toggle between upvoting and downvoting a
post without raising an error or having to manually remove the vote
first. If you upvote a post, then downvote it, the upvote is
automatically removed and replaced by the downvote.
Other changes:
* Tagging a post with `upvote:self` or `downvote:self` is now silently
ignored when the user doesn't have permission to vote, instead of
raising an error.
* Undoing a vote that doesn't exist now does nothing instead of
returning an error. This can happen if you open the same post in two
tabs, undo the vote in tab 1, then try to undo the vote again in tab 2.
Changes to the /post_votes API:
* `POST /post_votes` and `DELETE /post_votes` now return a post vote
instead of a post.
* The `score` param in `POST /post_votes` is now 1 or -1, not `up` or
`down`.
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.
