Check if spree route proxy responds to root_path

Some stores may not define a root_path under spree engine. Currently
only spree_frontend component does that.
Add product with track_inventory false via backend

Fixes #4309

Conflicts:
	api/app/helpers/spree/api/api_helpers.rb
	api/spec/controllers/spree/api/variants_controller_spec.rb
Remove useless currency.nil? check
Added an explanation to frontend unauthorized.erb