Ensure quantity is an integer before passing it to self.contents.add in Order#update_line_items
[api] Ensure total_quantity and display_item_total are available on orders/index and orders/show
[api] Expire taxonomies_controller#index in 3 minutes
Fix variant thumbnails not showing on frontend

The thumbnails for variant would not show on variant click. And the
product page opened without any visible thumbnails.

Fixes #3130