Scenario sets

To be reviewed commit by commit

Linked to https://www.agileplannerapp.com/boards/105200/cards/6053, to enable parsing scenarios in Smartdown, and exposing them as part of the Api::Flow object. 
I think it would be worth creating explicit Smartdown models for the Scenario objects (but still `OpenStruct` internally). Gives us more flexibility in the future, easier to debug, add tests, doc, etc.