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.

test/functional/posts_controller_test.rb

@@ -633,7 +633,6 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           assert_response :success
           assert_nil(response.parsed_body["md5"])
           assert_nil(response.parsed_body["file_url"])
-          assert_nil(response.parsed_body["fav_string"])
         end
       end
     end