Refactor DelegateBelongsTo to not touch db on startup
show the SKU when present on the line_item.variant

Fixes #4045

Conflicts:
	backend/app/views/spree/admin/orders/_shipment_manifest.html.erb
Remove code already placed in awesome_nested_set
Flush out association between payment and its method.

Change-Id: I7ae6a6071ad9959eacb48bda75061e58236e674f