Wave goodbye to Spree.routes

Say hello to Spree.mountedAt and Spree.pathFor.

Spree.mountedAt will tell you the relative path that Spree is mounted at.

Spree.pathFor allows you to make your own *magical* Spree URLs:

Spree.pathFor('/api/checkouts/R123456789')

No more Spree.routes mumbo jumbo
Remove Spree.routes file in backend
Use apply_coupon_code route helper in backend/adjustments.js
Always show 'Tracking' label if there's no label

This fixes a backend order_details_spec which was expecting to see the 'label' but it wasn't there