To edit the invoice's due date, we have several scenarios:

- A Payment Term is set. In this case, no extra field will be shown
  while in draft, as the standard flow allows us to edit the Payment
  Term directly, or delete it and set a due date manually. Once the
  invoice is posted, a new field will be shown in the view above the
  Payment Term, allowing the due date to be changed.
- No Payment Term is set, and the due date is set manually. In this
  case, the due date is editable while in draft, as with the standard
  Odoo flow. However, once posted, the field will remain editable.

(All the above are considering the user belongs to the security group
refered in "Configuration")
