[FIX] account: add invoice partner as follower

Feature was lost in the accounting-pocalypse merge.

Partner (customer) should be added as follower when the invoice is
posted, previously open.

See https://github.com/odoo/odoo/commit/beaa30a3d1843de43a45f419bfbc1bfa7613a920#diff-f3712147f4b25231caa371bd83265dbeL1447



We also add the commercial partner now.

Closes odoo/odoo#34766

Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>