Draft support of filling Mistral workbooks (via Barricade.js)

Implement tree-like inputs structure, next things to come are:
* storing data into the model from inputs;
* data validation;
* converting data model into JSON/YAML;
* enable more complex relations between Mistral entities.

Change-Id: Ibda4b5b4856e9025a9d2fb9f0cdd449bd6a82303