[IMP] point_of_sale: Prevent backend sessions and orders creation

If your create an order in backend and you make payements, they remain opened while the POS session isn't closed, which may seens counter-intuitive. The payment in taken into account afterward and the reconciliation is made with the invoice (if applicable)

We shouldn't let people create POS orders from backend. And while we're at it, we should do the same with sessions. They're opened and closed from the dashboard.