When sending the same survey to the same partner, the system detects it and
allows the user to choose between sending a new invitation or resending the
old one so that the user can continue the survey that might already be ongoing.
The detection was not working anymore and this fixes the problem.

Technical note: _compute_existing_partner_ids was intersecting record set
partner_ids (the one currently added) and a record set of partner for which an
invitation has already been sent. The intersection didn't work because
partner_ids relation was not completely set and instead of id, had "origin" id.
Comparing id using a filter solves the problem (using the trick that .ids,
return also the origin ids).

Task-3087702

closes odoo/odoo#108448

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>