[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
Add endpoint to retrieve a current user's orders in the API.

Returns 401 if the user is anonymous.

Fixes #4022
foo
remove the whole admin order #show redirect and go to edit instead.

Fixes #4015
Ensure payment_state is *really* set to credit_owed when an order is cancelled

Fixes #3711
avoid blank fields on stock management + images screens when variant has no SKU

Fixes #3979
Fixes #3650
Display a "Name on card" input on payment step

It creates a name column on spree_credit_cards table to insert the data
instead of using the old first_name and last_name fields, fixes #2486
Move payment partial from core to frontend

Plus make it use the name attribute instead of the deprecated first_name
and last_name
Add CHANGELOGs entries for payment name on card feature
there is a conflicting migration in 2-1-stable branche (8de75a5d3d519c5498843edda983702852caaade)
it's order_id ofcourse!
Make Stock::Package more aware of units structure

`add` and `find_item` now might receive a variant and a line item for
cases where they both don't refer to the same product
Grab shipment line items via inventory units

The previous implementations doesn't make sense anymore on the current
split shipments scenario. Can't see a reason why tracking inventory
levels config should influence whether or not the line item should be
returned