[FIX] account, base: Creating contacts from their parent or setting their parent

Bug in 13.0 regarding company_id field of partners when creating them from
their parent contact or when setting their parent.
The field 'company_id' of res.partner is set by an onchange on 'parent_id'
to that of it's parent; unfortunately the field is readonly if parent_id
is set and not set to force_save.
Furthermore, creating a child partner from the main one (in the 'Contact'
tab of the main partner form view) does not follow the same behaviour
(company_id is unset in that case, meaning that children don't have
the same company as their parent).
Since the company_id is already set by an onchange when we change 'parent_id'
and set to readonly in that case, I assume the expected behaviour is actually
that children partners should have the same company as their parent by default.

Fine tuning of 7b49f583

opw:2176384,2167106

closes odoo/odoo#43539

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>