[16.0][IMP][WIP] account_invoice_ubl: add support to self-billing + various fixes

[jbaudoux]

Allow to generate self-billing invoice and credit note in UBL.

https://docs.peppol.eu/poacc/self-billing/3.0/bis-sb/

cc @lmignon @rousseldenis @sbidoul

Also fixed in:

• base_ubl:
  • contact : do not declare if empty, an element cannot be empty
  • taxes : use invoice line taxes instead of product taxes
• account_invoice_ubl:
  • due date : must be declared as we always declare an amount residual (no matter who has to pay)

[lmignon]

IMO, this addon should depend on account_invoice_supplier_self_invoice. Personally, I would have preferred the term self_billing to be used in the name of this existing module and for the fields defined in it instead of self_invoice, but despite this, it would seem logical to me to depend on this module.