Merge branch 'spree/admin/root_controller' of https://github.com/CodeCantor/spree

Conflicts:
	backend/config/routes.rb
Fix admin root path, and update syntax.
refactor order content model

fixes #5545
Restore accessor method `package.order`, now computing it based on the first inventory_unit.

Added specs to detect the regression.
This caused significant problems in spree_active_shipping, which relies on `package.order.ship_address`

fixes #5546
Fix migration issue where default column doesnt exist yet.