erp5_oauth2_authorisation: get user by id instead of login

That way, we do not require creation of a login at all to use OAuth.
5 jobs for kd-portal-oauth in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.PSA.UnitTest-Titouan.KDPortal

01:24:07

failed ERP5.PSA.UnitTest-Titouan.KDPortal

00:57:51

failed ERP5.PSA.UnitTest-Titouan.KDPortal

00:48:04

failed ERP5.PSA.UnitTest-Titouan.KDPortal

01:26:35

passed ERP5.PSA.UnitTest-Titouan.KDPortal

00:48:19