llegal mix of CSF default export and storiesOf calls in a single file: ./documentation/documented_stories.js

Since the upgrade to Storybook 5.3, a bunch of errors are being logged to the console in development:

Illegal mix of CSF default export and storiesOf calls in a single file: ./documentation/documented_stories.js

This seems to be caused by the introduction of a new format for writing stories: Component Story Format.

Let's come up with a fix.