FEATURE: add support for group members visibility level (#8004)

There are 5 visibility levels (similar to group visibility)

public (default)
logged-in users
members only
staff
owners

Admins & group owners always have visibility to group members.

app/serializers/user_serializer.rb

@@ -143,7 +143,7 @@ class UserSerializer < BasicUserSerializer
 
   def groups
     object.groups.order(:id)
-      .visible_groups(scope.user)
+      .visible_groups(scope.user).members_visible_groups(scope.user)
   end
 
   def group_users