Allow to restrict visibility of users

Description

At the moment, everybody can enumerate the registered users of the gitlab instance (e.g. by going into 'Settings' of a project and see the memberlist, or when searching issues and looking at the assignee list) and get a complete list of the users of the gitlab instance. It would be nice when this would be prohibited for external or anonymous users.

We would like to provide a gitlab server to communicate with our customers. But we do not want to reveal to customer A that we have a relationship with its competitor B too (or at least, we do not want to make it obviously).

Proposal

• APIs which enumerate users should be available to non-external users only

• APIs which search for users should be unavailable to anonymous users

For my usecase, I would like to add further restrictions, but they might break existing functionality:

• APIs which search for users or return information about them (e.g. the profile page) should be unavailable to external or anonymous users