Add #in_stock? method to Spree::Variant

As proposed by @jdutil in spree/spree_wishlist#37

[Fixes #3163]
Split line items across shipments
- Easily create new shipments from existing shipments
- Move items between shipments on an order
Fix backend order split shipments action

When moving an item to a existing shipment from the same stock location
it would make two requests to remove items from the same shipment.
Instead it needs to remove from one shipment and then add the same
quantity to another shipment

[Fixes #3161] [Fixes #3162]
Fix build specs and split shipment edit button [Fixes #3168]

They were mostly failing due to a js error and shiping labels

Conflicts:
	core/spec/models/spree/shipment_spec.rb
Ensure correct classes on stateSelect, stateInput elements in address
spec

Fixes #3165
Namespace navigation tab translations so that they don't clash with translations such as en.spree.orders.show

Fixes #3133