Function change_visible 

pub fn change_visible(c: &str, actor_n: usize, hidden_n: usize) -> String

SQL guard: the change aliased c is visible to the actor bound at actor_n (uuid) whose hidden groups are bound at hidden_n. Visible to administrators, its author, its reviewers and anyone who sees every affected item of a change that has items (an empty draft is therefore visible to its author, reviewers and admins only).