Add total on hand to variant RABL

added test coverage for new rabl fields

fixes #5593

Conflicts:
	api/app/views/spree/api/variants/small.v1.rabl
Skipping the set_current_order before filter on the cart link action as this should use the simple_current_order helper only

fixes #5605
Don't load the store if we don't have to - minor optimisation but every bit helps
fixes #5606