Add support for unrecognized cells

[blink1073]

The classic notebook uses UnrecognizedCell when it encounters a cell type that is not in nbformat.

[williamstein]

Just saw this discussed during triage. I could imagine this coming up when somebody works with a notebook using some extension that supports a new cell type, and then that notebook can't even be opened without the extension.

For what it is worth, this exact thing (show a warning instead of the cell) is supported in cocalc, and it seems like a reasonable thing to support: