group_everyone should exclude guest users

[lars-becker]

We used the everyone group when we wanted to share information with any member of our organisation. We now started to use the guest application for people outside of our organisation with whom we regularly share files. The problem is: they are also part of the everyone group which surely fits to the semantics of "everyone", but which oftentimes will make no sense sense since its not possible to differentiate between internal users and external like its possible when sharing with links and which will also introduce the potential for data leakage since every user can create guest users.