refactored invoices to allow different invoices
with the addition of multiple shipments in spree 2
theres a need for different invoinces per shipments
specially for international shipments where customs
needs to see an invoice for the shipment
use shipment manifest to unify product display
calculate per shipment invoices subtotal
fix item quantity and added shipment buttons
remove inventory units w no line items
move to line items instead of manifests
cleanup comments and remove unused parts