Make active meetings accessible for guests

[bastianjoel]

related to OpenSlides/openslides-client#3955

enable_anonymous might not be sufficient anymore as we want an overview of all public meetings on orga level. Therefore we need to be able to decide on the orga level login page if 1. there is a meeting which is public, 2. which meeting to redirect to, 3. are multiple meetings available and therefore the dashboard need to be displayed.

Another option would be to relax active_meeting_ids restrictions which we would need to discuss.