Split line items across shipments
- Easily create new shipments from existing shipments
- Move items between shipments on an order
Fix for line item shipment total price display
[backend] Updated CHANGELOG
- Split line items
- Fix line item total on shipments
Fix users rule promo association

Also add an association_foreign_key config because it assumed that every
Spree.user_class would return User which might not be always the case.

[Fixes #3145] [Fixes #3149]
Load promotion rules after app initialization

So that they can make proper use of `Spree.user_class` which is only
defined on the app spree.rb initializer