[FIX] account: only put internal users as followers of vendor bills created by emails

When the pdf of a vendor bill is sent to the alias of a journal,
all the partners that match one of the email address present in the mail thread
are added as followers of the newly created record.
This is a problem since the vendors are actually not concerned by the internal
work that is done, only internal users are.
As a result they get spammed by useless emails, and complain to Christine.
It was put in commit a4df9f8c despite not appearing in the task description.

We filter the partners to only keep the ones that belong to internal users.

opw 1958729

closes odoo/odoo#32110

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>