Use flush effects to custom apply css-in-js solution to app. Re-introduce flush effects to app-render, and remove default support of styled-jsx in `app/`. So that users will choose their own css-in-js solution if they need any customization. styled-jsx won't appear in client bundle if you didn't use it.
For now we have to inject the initial styles before `</head>` to avoid hydration errors. Later on we can remove this once react can handle it.
- [x] inject styles before end of head element
- [x] add tests
