Implement a cornerstone toggle component

[IreneStr]

When you enable the feature flag, you'll see an empty sidebar (or a partially filled sidebar if other PRs have been merged in the meantime). Add the above component to the sidebar. For the order of the sidebar content, see https://user-images.githubusercontent.com/6073772/40122626-87e97e26-5924-11e8-8c82-ac5a7ead9b00.png.

To implement the component, update the Gutenberg dependency in the project (if that hasn't happened yet), and reuse their components. The component(s) you need to copy/use, is already used in Gutenberg's document sidebar: