Only create an order when adding a product to cart

Everytime user was authenticated an order was created by visiting any
page in frontend (if none were already associated with the current user)

Fixes #4136

Conflicts:
	frontend/spec/controllers/spree/orders_controller_spec.rb
show the SKU when present on the line_item.variant

Fixes #4045
Refactor DelegateBelongsTo to not touch db on startup