Add total on hand to variant RABL

added test coverage for new rabl fields

fixes #5593
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
Move default price to a concern.

fixes #5505
Include classificaiton position with taxons, undo of #5560

Spree has really useful classification position scores that are adjustable in the Admin. Let's expose this over the Product payload in the API and next taxons under there.

fixes #5609
Test latest 2.1.5

Conflicts:
	.travis.yml