In Jupyter notebooks, read strings stored in the raw as either "string" or "list<string>" more consistently
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.
Created a notebook with a code block represented on disk as a single string, rendered a diff from it.
Differential Revision: https://secure.phabricator.com/D21434