Public Access: Add a new setting for global activation

[rrenkert]

As stated in OpenSlides/openslides-client#4204 we need a new setting for the superadmin to activate/deactivate the public access globally on orga level.
The setting should prevent publishing a meeting if this option is set to false.
The default should be false and if toggled from true to false all published meetings should be unpublished.

[luisa-beerboom]

Discussed with @rrenkert that the setting should not automatically unpublicise public meetings, but that their public status should instead be hidden in the same way as delegations do if they are turned off.