Improve UX save dni at Party form

[lukio]

We have the field TIPO DOCUMENTO. We could improve the set_vat_number so if TIPO_DOCUMENTO says DNI, the value saved to the system could be ar_dni instead of ar_cuit.

Perhaps, instead of improve set_vat_number, we could add a different field to saved the identifier from the header.

[lukio]

If we reuse the field, I think it will break account_invoice_ar :/