Restore support for the legacy notebook mechanism from 0.16

[jleibs]

What

• Resolves: Restore legacy notebook API #7012

Some systems are still incompatible with the new anywidget based notebook approach.

Keep the old way of doing things around for the time being. This could probably be improved with new technology we've created, but this is a verbatim copy of how things worked in 0.16.

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• I have read and agree to Contributor Guide and the Code of Conduct
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• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

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