Fix ajax calls for adding and deleting order item meta from the backend.
Check products are purchasable when restoring cart

If WC_Product::is_purchasable() is only called when adding a product to the cart,
it's possible to purchase products that aren't purchasable (but were when they were
added to the cart).
Add WC_Cart::get_cart_for_session()
Implement order status update endpoint

Closes #4161
Add action for plugins to load other API resources

 Part of #4055
Perform core authentication at 0 priority

Makes it easier for plugins to override the core API authentication

Part of #4055
Load admin profile class when editing a user
Fix permissions typo

Part of #4055
Specify 2 decimal places for woocommerce_format_decimal()

Part of #4055
Remove schema link
Remove PUT/DELETE endpoints

Will be implemented in 2.2

Part of #4055
Add total sales property to products response
Normalize response data types

Part of #4055
Fix incorrect total results number for pagination headers
Add permissions for count endpoints
Add total customers count to sales report

Part of #4055
Brace all the things
Updated Danish Language files
Merge pull request #4184 from maxrice/api-order-status-update-issue-4161

Implement order status update endpoint
Merge pull request #4185 from maxrice/rest-api-fixes-issue-4055

Various fixes for REST API