Promote visibility level helpers from Group to Namespace

What does this MR do?

Promotes visibility level helpers from Group to Namespace

Are there points in the code the reviewer needs to double check?

Is this going to have any ill effects? From what I can see, non-group namespaces all have a visibility level, so I'm hoping this is fine.

Why was this MR needed?

500 error in Gitlab EE with personal projects and namespace plan checking enabled

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2950