[IMP] account: add a readonly group

Task 2092079
Accounting firms that want to give access to their customers avoiding
mistakes and risks will love this profile that can't do anything
wrong... Maybe as well as companies auditors..?

closes odoo/odoo#39860

Related: odoo/enterprise#6576
Signed-off-by: Quentin De Paoli (qdp) <qdp@openerp.com>