Enable new Storybook build options

[daneah]

The problem

We want the DX and the consumer experience for Pharos to be as fast and cheap as possible.

The solution

Enabling story store v7 and babel v7 will bring improved performance through code splitting and pave the way for future build improvements to make the DX better (blog post).

Additional information

I had tried implementing these things in (this commit) but it was failing the build as shown here; probably something we can get past with some configuration updates.