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/models/post.rb

@@ -762,12 +762,11 @@ class Post < ApplicationRecord
       update_column(:fav_count, fav_count)
     end
 
-    def favorited_by?(user_id = CurrentUser.id)
-      fav_string.match?(/(?:\A| )fav:#{user_id}(?:\Z| )/)
+    def favorited_by?(user)
+      return false if user.is_anonymous?
+      Favorite.exists?(post: self, user: user)
     end
 
-    alias is_favorited? favorited_by?
-
     def append_user_to_fav_string(user_id)
       update_column(:fav_string, (fav_string + " fav:#{user_id}").strip)
       clean_fav_string! if clean_fav_string?