Allow the Jupyter engine to elect to emit diffs, and emit Jupyter documents as blocks

Summary:
Depends on D20832. Ref T13425. Emit Jupyter notebooks as diffable blocks with block keys.

No diffing or proper inlines yet.

Test Plan: {F6888058}

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20833