Support for private GitLab Pages

Description

Right now it seems that Tor's GitLab instance does not have an option to have a private GitLab Pages.

Having this feature would be handy for those like me who have a (still) private repository that needs a private GitLab static site.

Steps to reproduce

1. Access a private repository on Tor's GitLab and on GitLab.com.
2. Go to "Settings > General" and expand "Visibility, project features, permissions" on each repository.

Current result

On GitLab.com we have an option for controlling Pages visibility:

On Tor' GitLab we don't.

Expected result

Having an option for controlling Pages visibility on Tor's GitLab.

Suggested fix

GitLab Pages access control docs says:

[...] If you don’t see the toggle [Pages visibility] button, that means it isn’t enabled. Ask your administrator to enable it.

I could not check whether the current GitLab version at https://gitlab.torproject.org does not have this option, neither check if it runs the same version from GitLab.com (probably not), but it's possible that this feature is already available in recent GitLab versions.

I'm also unaware of the performance, labor and complexity impact of this request.

Thank you :)