Use find_by_id instead of find when looking for user

Related issue: https://github.com/NoamB/sorcery/issues/376

So I've changed this method, but it actually does not change library behaviour - Sorcery should not crash on `login_from_session` because it rescues `NotFound` errors. Anyway, this way method is simpler, and when we merge https://github.com/NoamB/sorcery/pull/509 I'll be able to simplify it even more (no need to return false then)