Upgrade to `styled-components` v4

styled-components v4 was just announced, and it looks really good:

• 25% faster on mount
• really good new APIs like the new global styles, and as
• ref support
• faster styled()
• a bunch more

It's still in beta, but I think we should try upgrading. It seems like it'd be a huge improvement.

Some things that will need to be updated:

• We use injectGlobal in a few places, needs to switch to the new global API
• use ref instead of innerRef in a few places
• use styled() instead of .extend
• For the Heading component, we can use the new as prop, instead of withComponent

This would be a great first issue for new contributors :) would give you a tour of the React codebase, without being too hairy.