Add model for Simple Smart Answer

This adds `SimpleSmartAnswerEdition` to the line-up of content models. This is based upon the work done by @henryhadlow and @jackfranklin with a [smart answer builder](https://github.com/alphagov/smart-answer-builder).

A simple smart answer is composed of a hash of `nodes`. It also has a `body` - which is used as the introductory text on the first screen.

There is validation on the nodes hash to make sure that each node is itself a hash, and that each node has a title. (In the future, there will be more validation to make sure that there are no dead-ends or circular flows.)