Show coop agreement id always in Partner form

CHANGELOG:

### Changed
- [#620](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/620) Make field coop agreement visible for edit.

Related card: https://trello.com/c/oNsFr4uf/772-test-de-transici%C3%B3-destats-del-soci

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With this change, we allow the agents to change the info of an old member to coop agreement.