Prior to this commit, subtasks that were added (from task templates) as
part of a visit (from sales orders), were having their partner_id set to
False by a built-in Odoo _compute_partner_id method.
This fix (and previous commits) introduce a test that correctly failed
when this behaviour was broken as well as modifications to the behaviour
of sale order lines and tasks to ensure that all tasks created from a
sale order for an FSM project correctly get their partner_id set to the
sale order's delivery address.
