Link pack contracts after creation

⚠ Depends on https://gitlab.com/coopdevs/migrated-projects/odoo-somconnexio/-/merge_requests/897 ⚠

CHANGELOG:

### Changed
- [#900](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/900) Link pack contracts after creation.

Related card: https://trello.com/c/H8mTm47S/1442-creaci%C3%B3-dels-contractes-de-pack-a-odoo

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