Add discovery_channel to Partner computed from SubscriptionRequest
CHANGELOG:
### Added
- [#647](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/647) Add discovery_channel to Partner computed from SubscriptionRequest
Related card: https://trello.com/c/pf31k8Jr/957-migraci%C3%B3-del-camp-com-ens-has-conegut-de-tryton
We need the field Discovery Channel at Partner level to use it in the commercial area.
Compute this file from the newest Subscription Request in done or paid states and related with the partner.
Edited by Pelayo García