[api] Add eager loading for stock items, products, images and prices to variants#index
Dont update shipments and payment in order cart

Adding a product to cart on a fresh spree install currently runs 41 SQL
queries. This change makes the POST orders/populate request run 34 SQL
queries.

So each time the store needs to run `order.update!` and the order is not
completed yet we run 7 less queries. That could make a big difference
when lots of promotions are involved.

Fixes #3513
Drop spree_mail_methods table

It's no longer used since Spree 2.0

Fixes #3511
[backend] Add _spec suffix to stock_transfer test