[14.0][FIX] mail_activity_team: Manually keep `user_id` in sync with `team_user_id`

[ypapouin]

Ensure that the built-in user_id field value is in sync with our field team_user_id before triggering create, otherwise the framework will try to resolve the inverse value by itself and will call write before create has ended. That leads to multiple side effects like sending two assignment notifications.