update automatic visit/subject importer

• every subject that does not have a default appointment place should be assigned one
• country should be filled in by Luxembourg (configurable)
• configuration of the importer should be in GUI
• there should be possibility to have more than one source (impossible for now due to django_cron data model)
• automatic import/export should be allowed only from predefined folder inside smasch (to prevent overriding/exposing system information)