favorites: remove is_favorited attribute from post API.

* 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.

app/presenters/post_presenter.rb

@@ -131,7 +131,6 @@ class PostPresenter
       "data-source" => post.source,
       "data-uploader-id" => post.uploader_id,
       "data-normalized-source" => post.normalized_source,
-      "data-is-favorited" => post.favorited_by?(CurrentUser.user.id)
     }
 
     if post.visible?