Cashier cannot link a customer in orders page

[khaira777]

If an order was first created without linking to a customer, later if cashier wants to link it to an existing or new customer, there is no option.
Similar to #716, cashier should atleast have the option to do so and it can be escalated to manager/owner.
Need to ensure this is not misused so apply/add this functionality accordingly, please consider edge cases or use case.