Return all projects and groups for user with read_admin_groups and/or read_admin_projects

What does this MR do and why?

Regular users with custom admin roles granting read_admin_groups and read_admin_projects should be able to view all groups and/or projects like instance admins and auditors.

This MR updates GroupsFinder and Project.visibility_levels_for_user (used in ProjectsFinder) so they return all groups/projects when the current_user is a regular user with custom admin role granting read_admin_groups and/or read_admin_projects.

References

For Permission to view custom attributes (or groups... (#534449 - closed).

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