Allow changing to SC cooperator agreement in partner view

CHANGELOG:

### Changed
- [#838](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/838) Allow changing to SC cooperator agreement in partner view

Related card: https://trello.com/c/hxvXAYNu/1308-no-podem-escollir-somconnexi%C3%B3-per-apadrinar

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