Enable Code Folding by Default

Some users that I support love having the [Code folding](https://github.com/ipython-contrib/jupyter_contrib_nbextensions/wiki/Codefolding) extension installed.


This seems like something we could support out of the box with jupyter notebook instead of having an extension. I'd certainly love having this even for beginners to help deal with unwieldy large notebooks.

Start by reviewing the linked Code folding extension and its behavior in large notebooks. Compare that behavior with the existing Jupyter Notebook interface and identify the relevant frontend entry points before proposing scope. Done means code folding is supported by default without requiring the extension, with the expected behavior documented and tested.

Written by the indexing model from the issue text.