* Remove the data-is-favorited attribute from post thumbnails.
* Remove the is_favorited attribute from the /posts.json API.
* Remove the fav_string attribute from the /posts.json API (only visible
to moderators).
* Change `Post#favorited_by?` to not use the fav_string.
Further addresses #4652 by eliminating the last places where fav_string
was used.