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).
Fix payments ordering when choose payment for risk analysis

Spree::Payment has ordering by created_at by default.
When displaying order risk analysis Spree tries to select last payment for order and uses ordering payments by "created_at DESC" and then select first record, but it does not work. So we need either to use reorder or to rely on default ordering.

fixes #5541
Fixes code snipped in preferences.md

It was not closing the block properly.

fixes #5542
HTTP Patch makes direct update_attributes call on Spree::Order

fixes #5544
fixes #5543
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
Fix migration issue where default column doesnt exist yet.
Fix "non_tax" scope for Adjustments

Needed to account for nulls
fixes #5550
Remove unused permit_attr call.
Fix variant price validation to allow nil

* The check_price validation already asserts presence, or not depending on
  the system config, and unnecessarily validations are done when
  the master variant price is not required.

Conflicts:
	core/app/models/spree/variant.rb

fixes #5552
Fix pagination in /api/taxons/products
fixes #5554
Fix order / adjustment presence in adjustments

* Fix admin action producing adjustments without direct order reference.
* Add validation for the presence of adjustable
* Adjustments without an adjustable item do not make too much sense
* Add migration to fix old adjustments without direct order reference

fixes #5556
Don't trigger the reimbursement mailer until the reimbursement has been persisted

fixes #5560
Fix spec.
Fix the cookie domain used for google analytics

* A change needed to make the google analytics integration compatible
  with the universal analytics engine. Should also work for the
  traditional code.

fixes #5565
Fix validation to skip performing a query if the field is blank
fixes #5565