[contract] reactivate the closed contract at the SO's confirmation

- Task: https://www.odoo.com/web?debug=#id=6527&view_type=form&model=project.task&action=327
- Pad: https://pad.odoo.com/p/r.hhgH3UWxNoWzXLBT
case where `account.analytic.account` not loaded is not handled, so just use `self.pool['account.analytic.account']`. Same below