[api] Return attachment URLs using the avaialble styles

This is so an API endpoint can render all the styles of an image, not just the Big One.
[api] Add CHANGELOG entries for products+images API changes
Cache product API responses for 3 minutes to make things like Backbone cache them
Add spot in spree application where JS templates can be yielded to

By default, this does nothing.
[api] Use order.contents.add() when updating line items

For example, when an order has a line item with variant_id=3 and quantity=1, when a line item parameter comes into this action with the same variant id, it should increase that line item's quantity, rather than creating a new line item
[api] Return display_item_total in orders/show
[api] Use real world example for line_items_attributes in OrdersController#update action
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
[api] Remove caching of products (index+show) + taxonomies (index) actions
Return Cache-Control headers for all API requests
Correct engine path.