Add internal name for stock locations.

Includes light refactoring of stock_locations/index.html.erb from Ryan Bigg.

Fixes #3532
Apply tax charges to line items for an order, rather than the order itself
Comment out choose best promotion adjustment for now
Add migration for re-arranging adjustment fields
Adjustment.shipping scope now picks up adjustments for shipments
Default tax calculator now calculates taxes for shipments and never for orders
Order total now comprises of order.item_total + order.shipment_total + order.adjustment_total

This will need to be refined later to be:

(order.item_total - order.item_adjustments) + (order.shipment_total - order.shipment.adjustments) + order.adjustment_total
One shipment can now have many adjustments

Although we don't make any use of this currently... it will be used later on for discounts + credits
Shipments no longer create a single adjustment when just one value in a field on the record will do
Remove all tax adjustments when recalculating tax for an object
Rename cost to amount on spree_shipments
Add /checkout/summary which gives a clearer breakdown on how an order's total comes into being
Only create an adjustment for tax rates if the amount is > 0
Define Adjustment.promotion association correctly
Remove code to set eligibility for adjustment from Adjustment class

I am not too sure that this is necessary, given that this seems to only be used to determine if promotion adjustments are eligible or not, and that's done where the best promotion is found, in OrderUpdater
Use amount, not cost, to compute a shipment in default tax calculator
Promotion adjustments are now created on a per-line-item basis
Order#ship_total now sums up all shipments' amounts
Remove promotion_credit_exists? from Order, as this is now on LineItem
Use SQL query rather than Ruby summing in Order#promo_total