Provide an option to disable the ability to create Public Projects

Summary

We don't currently provide the option to remove the ability to create Projects with public visibility across the GitLab instance.

We do currently provide the ability to restrict the public level for non-admin users but not for admins too

Proposal

Should we provide the ability via an option in the Web Interface or configuration to disable public project creation across a GitLab instance for all users and groups?

Relevant Zendesk Ticket

https://gitlab.zendesk.com/agent/tickets/26542

/cc @JobV