Undo-redo support in notebook

[epatters]

Add support for undo/redo in the notebook. This functionality ought to ship with Automerge but it does not yet (despite appearing as an experimental feature in early versions of automerge-classic).

Here is a recent experimental implementation from the community: https://github.com/onsetsoftware/automerge-repo-undo-redo

[olynch]

Given that we get out immutable references from automerge, we should be able to just keep a stack of versions around. I implemented undo-redo manually in Semagrams, and it wasn't so bad, but there are some subtleties; I'll self-assign for this so I can use what I learned.

[epatters]

I'm hoping that the Automerge team can help us with this feature. They're open to abstracting parts of the infrastructure built for Patchwork into a standalone, reusable package, though that will take some time.

[kasbah]

Closed by #1175