FEATURE: add support for group visibility level

There are 4 visibility levels

- public (default)
- members only
- staff
- owners

Note, admins and group owners ALWAYS have visibility to groups

Migration treated old "non public" as "members only"

app/serializers/basic_group_serializer.rb

@@ -5,7 +5,7 @@ class BasicGroupSerializer < ApplicationSerializer
              :display_name,
              :user_count,
              :alias_level,
-             :visible,
+             :visibility_level,
              :automatic_membership_email_domains,
              :automatic_membership_retroactive,
              :primary_group,