create a single global model and adapt the old StateModel logic

[lorenzo-cavazzi]

After discussing and trying a couple of solutions for addressing #623, it seems reasonable to do the following

• create a single global model in the entry point src/index.js and pass it down to components requiring access to sub-models or to the redux store
• move the sub-schema and re-factor the <component>Model as <component>Coordinator (start with notebooks as first example)
• adapt the functions mapping the redux state to react properties