Add a disenrollment reason field to fulfill when finishing a cooperator's membership
CHANGELOG:
### Added
- [#662](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/662) Add a disenrollment reason field to fulfill when finishing a cooperator's membership
Related card: https://trello.com/c/QnEHil4q/996-nous-camps-a-baixa-de-s%C3%B2cia
Edited by Pelayo García