Add offtrack option for all stories

[adityasohoni]

Stories currently have 2 paths. A happy path and a sad path. Add another path if the user changes the topic midway in the story.
Theoretically, a user can switch context from an intent to any other intent.
But in practice, there are some pairs of intents that are similar and the user often switches the context here.
So set up paths that take care of these off-track context switches.
https://rasa.com/docs/rasa/2.0.x/writing-stories#handling-context-switching