Preview document moves

Requires folders_view_enabled. Computes effective user-level and durable Team/Tenant audience increases and decreases for an atomic move or unfile. Each change includes document_uuid, document_name, audience_type (User/Team/Tenant), audience_uuid, audience_name, before_level, after_level and direction; User rows also include user_uuid and user_name. Layouts retain tenant-wide view access.