Default credit card per user

When an order is completed, we save the credit card used as the user's
default credit card. When creating a new order, we use an existing
credit card (if it exists) and attach it to the order before reaching
the payment state.

Fixes #5148

Conflicts:
	core/app/models/spree/credit_card.rb
WIP Expedited exchanges

When a return authorization is saved
Given a return item has a requested exchange
Then an exchange shipment for the exchange items for the return authorization are generated immediately

Given an exchange shipment has been generated pre-return
Given a customer has not returned their item after X days
Then create a new order with the exchange shipment and capture payment for the full amount of the items