LNPayerIsPayer field in the invoice proposal should not be editable from the front-end

Introduction

LNPayerIsPayer is an internal field that evergreen uses to identify whether an invoice proposal is multi-payer.

Although evergreen does not expose this field in the front-end, users could add it to the invoice proposal screen by means of personalisation. As LNPayerIsPayer is an internal field and editing it may cause unexpected issues in the billing process, it has been made non-editable.