favorites: order /favorites.json by id.

Order https://danbooru.donmai.us/favorites.json by favorite ID instead
of by post ID. This way you can get a feed of recently added favorites.
Previously this wasn't possible because we didn't have an index on
favorite ID.

app/models/favorite.rb

@@ -18,7 +18,7 @@ class Favorite < ApplicationRecord
       q = q.for_user(params[:user_id])
     end
 
-    q.order(post_id: :desc)
+    q.apply_default_order(params)
   end
 
   def self.available_includes