Check for parts stock items instead if product is assembly
Display product bundles on backend orders UI
Remove before_payment code from CheckoutController

Otherwise it removes the product bundle items on the order because
they're not included in the shipment (unshippable)

There's a PR for this on spree/spree but I'm not sure it
will be accepted, see https://github.com/spree/spree/pull/3193
Group inventory units by line item on shipment manifest

Update Package::ContentItem to also have a line item.
Resolve conflich between individual product and bundle part

Spree should know whether the inventory unit is associated with a
product sold individually or with a part of a bundle.

We probably need to also add a part column on the InventoryUnit table so
that this behaviour is persisted at the unit level doesn't depend on the
bundle product state which might change in the future
Scope uniq line item assemblies only
Fix case where Coordinator doesn't return proper packages
shipment.manifest returns all units with proper quantities

Spree restock and unstock items based on the units returned by
shipment.manifest so if it returns wrong the error follows to the stock
item on hand quantity
Account for situation units don't have a line item
Improve test coverage

Remove repeated spec and ensure shipment manifest behave as expected on
this extension
Group shipment units per line item on frontend
Removed old pending spec.