MOD assign context company_id in partner creation

Related to this card: https://trello.com/c/PkBCQRg0/198-en-crear-un-nou-partner-aquest-es-crea-sense-assignaci%C3%B3-de-company-i-per-tant-queda-a-la-vista-de-totes-les-ce

On create new partners, the field company_ids must get the context company_id as default value. If not any new partner remains visible across all the companies.