Add upvote and downvote buttons beneath thumbnails on the post index page.
This is disabled by default. To enable it, click the "..." menu in the top
right of the page, then click "Show scores".
This is currently a per-search setting, not an account setting. If you
enable it in one tab, it won't be enabled in other tabs.
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.
