Add Spree::Admin::RootController for redirecting purposes.

Currently, is really difficult to redirect to a different tab when the
user has logged in if said users cannot at least :admin, :index
`Spree::Order`s.

This commit adds a controller that handles `/admin` route so anyone can
easily redirect to another page (or implement any other logic).
refactor order content model
fixes #5545
Merge branch 'spree/admin/root_controller' of https://github.com/CodeCantor/spree

Conflicts:
	backend/config/routes.rb
Fix admin root path, and update syntax.
fixes #5346
fixes #5345
Revert "refactor order content model"

This reverts commit 7da45f2ca0706417d132d8da6d133908dece8255.
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