Following the removal of read access on ir.model (odoo/odoo#69120), the mail.activity.type model was not accessible to non-admin users due to the res_model_id many2one field. Before this commit, a project user could not access the Activity Type menu. Convert it to a selection field with the selection values being computed in sudo. closes odoo/odoo#74981 Related: odoo/enterprise#20214 Related: odoo/upgrade#2734 Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>