add storybook stories for ActionItem and Toggle

Created by: sqs

Storybooks are a way to demo the various important states of UI components. @ijsnow originally added some for browser ext code, and they're quite useful. This adds some storybooks for shared UI code. These don't run in CI (yet; we can use storyshots in the future) and nobody else needs to use them yet. I'm just merging them so people interested can use them and add new storybooks if they choose.