When rendering Jupyter notebook diffs, split code inputs into individual blocks

Summary:
Ref T13425. Currently, code inputs and all outputs are grouped into a single block. This is fine for display notebooks but not great for diffing notebooks.

Instead, split source code input into individual lines with one line per block, and each output into its own block.

This allows you to leave actual line-by-line inlines on source, and comment on outputs individually.

Test Plan:

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20840