Grant registry auth scope catalog to GitLab admins

Zendesk: https://gitlab.zendesk.com/agent/tickets/56038

A customer would like to use the Docker registry API for integration purposes. GitLab restricts access to the catalog because it could lead to information leakage - which projects exist, etc. However, we should consider allowing access to this API for GitLab admins. In this case there's no risk of information leakage.

This would allow GitLab admins to use the full power of the Docker API when using the registry bundled with GitLab.