Import cells in model notebooks

[epatters]

When it makes sense, notebooks should have a new cell type for imports. Minimally this should be the case for model notebooks, but eventually also for diagrams (available) and instances (not yet available).

A first cut at this feature would enable a model notebook to import another notebook by specifying its ref ID (a UUID), presumably by copy-paste. This feature should ultimately be paired with

• Look up documents by name and type #378, to look up models by name
• Versioning documents #380, to optionally specify a fixed version of the model being imported