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