[api] product responses now contain master variant in its own key, separate from all other variants

This makes it easier to get to the master variant:

product.master - Definitely returns the master variant.

vs

product.variants[0] - This may not return the master variant!
Add ability to resort products ordering within a taxon

Fixes #4023
Add missing save_my_address translation
Spree::Product.in_taxon will now return products ordered by their position within the taxon
Revert back to using Product.in_taxon as the scope for TaxonsController

This fixes frontend's taxons_spec:20
[api] Always include products key in products/index results
[api] Remove unused taxon/products view
[backend] Show message when there's no products to sort