refactor as_story_string in StoryStep RasaHQ/roadmap#263
refactor as_story_string in StoryStep RasaHQ/roadmap#263

RasaHQ/rasa_core,RasaHQ/rasa_nlu,RasaHQ/rasa_nlu,RasaHQ/rasa_core,RasaHQ/rasa_core,RasaHQ/rasa_nlu