add views, sale_order.py file for backend promotion and security or access control

addons/promotion_modul/__manifest__.py

addons/promotion_modul/models/sale_order.py

addons/promotion_modul/security/ir.model.access.csv

...ns/promotion_modul/views/sale_order_promotion_applied.xml

addons/promotion_modul/views/sale_promotion_rule.xml