Add a very rough, proof-of-concept Jupyter notebook document engine

Summary:
Depends on D19252. Ref T13105. This very roughly renders Jupyter notebooks.

It's probably better than showing the raw JSON, but not by much.

Test Plan:

• Viewed various notebooks with various cell types, including markdown, code, stdout, stderr, images, HTML, and Javascript.
• HTML and Javascript are not live-fired since they're wildly dangerous.

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19253