Simplify styled-components example (#5631)
Simplify styled-components example (#5631)

- use `enhanceApp` so that styled-components used in _app.js are server rendered
- call parent getInitialProps, fixes #5629
- return `styles`, making the render() method obsolete.

cc @mxstbr @probablyup
azukaru/next.js,flybayer/next.js,BlancheXu/test,JeromeFitz/next.js,zeit/next.js,zeit/next.js,BlancheXu/test,JeromeFitz/next.js,azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,BlancheXu/test,flybayer/next.js,flybayer/next.js,JeromeFitz/next.js