Allow representative as contract's invoice partner

CHANGELOG:

### Changed
- [#998](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/998) Example text

Related card: https://trello.com/c/CgzmLD2b/1558-canvi-titular-must-be-invoice-type

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We stopped using invoice type partners to use the main partner address (representative). The validation should work according to this decision.