* Make ChannelConnection.is_active nullable so it can be eventually removed * Replace traceback.print_exc() with logger.error * Make sure contacts ids are iterable when starting a flow * Remove USSD proxy model
* Make ChannelConnection.is_active nullable so it can be eventually removed * Replace traceback.print_exc() with logger.error * Make sure contacts ids are iterable when starting a flow * Remove USSD proxy model