"Authenticated Users" Virtual Group / Role

**Is your feature request related to a problem? Please describe.**
For some of our events we would like to make them visible to everybody with an account on our indico system (be it ldap, be it local to indico).

**Describe the solution you'd like**
I would like some sort of group / role that can be added to event (or category) ACLs.

**Describe alternatives you've considered**
We're currently using some ldap group that catches 95% of the people.

**Additional context**
The following topics might be of interest. At least these were the ones, I have found regarding the topic:

* https://talk.getindico.io/t/category-restricted-to-authenticated-users/346 suggests to create a dedicated module that maintains the group semi automatically
* https://github.com/indico/indico/issues/1415#issuecomment-57685191 mentions this as part of GDPR things
* https://github.com/indico/indico/issues/2281 looks related, also I don't understand the full scope
* https://github.com/indico/indico/issues/5165 was mentioned in the discussions and seems to be a superset of the requested feature

If the feature exists already, then please excuse me.

Start by reading the related discussions in issues 1415, 2281, and 5165, along with the linked forum topic, to define how an authenticated-users virtual group should interact with event and category ACLs. Review the existing LDAP-group approach and authentication model, then establish the supported behavior and acceptance criteria before implementation.

Written by the indexing model from the issue text.