chore: include utilities in Storybook

This MR includes utilities.scss in Storybook. This will let us use utility classes in stories so that we have a consistent of demoing our components.

It's worth noting that this file is generated by the bin/generate_utilities.js script which doesn't "watch" for changes, this means that, when adding a new utility mixin, yarn generate-utilities will have to be run manually if the utility is needed in Storybook. We might improve this in future iterations.