fix(Sales Invoice): get user confirmation to link customer with patient (backport #462)

[mergify[bot]]

Issue:- Sales invoice against a patient that not linked to any customer will not allow billing

• This change will pops up a confirmation message asking for link the customer that selected in Sales Invoice permanantly and allow billing

https://github.com/frappe/health/assets/89388830/9f49f5e0-c07e-40e7-b03a-28aae51eb754

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This is an automatic backport of pull request #462 done by Mergify.