In Jupyter notebooks, read strings stored in the raw as either "string" or "list<string>" more consistently

Summary:
Ref PHI1835. Generally, Jupyter notebooks in the wild may store source and markdown content as either a single string or a list of strings.

Make the renderer read these formats more consistently. In particular, this fixes rendering of code blocks stored as a single string.

This also fixes an issue where cell labels were double-rendered in diff views.

Test Plan:
Created a notebook with a code block represented on disk as a single string, rendered a diff from it.

{F7696071}

Differential Revision: https://secure.phabricator.com/D21434