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

Returns 401 if the user is anonymous.

Fixes #4022
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