Extract render_to_static_markup & render_to_string to React::Server module

[zetachang]

Since react v0.14, react-dom is extracted out as separated package. [1]

And with the latest v15.4.0 released, ReactDOMServer now shipped with its own copy of React reconciler as the release note stated [2]

Another thing to watch out for is that React DOM Server is now about the same size as React DOM since it contains its own copy of the React reconciler. We don't recommend using React DOM Server on the client in most cases.

I think it's time to extract out the ReactDOMServer API. React::Server will contains the original React.render_to_string & React.render_to_static_markup. Things to watch out:

• React::Server is an opt-in module, you should try using innerHTML if you previously using those API at client side. (which is suggested by React DOM Server shouldn't contain a separate copy of the reconciler facebook/react#8316 (comment).)
• The copy of react-dom-server will need to be provided separately, react/react-source will also be updated to support this.

[1] https://facebook.github.io/react/blog/2015/10/07/react-v0.14.html
[2] https://facebook.github.io/react/blog/2016/11/16/react-v15.4.0.html

[zetachang]

Given that react-rails is not updated with this release yet, the actual work will start after v15.4.0 landed in react-rails master.

[zetachang]

So react-rails has shipped the latest version with the corresponding reactjs update.

And the proposed changes will be

1. react/react-source will still work as before, but a deprecation warning will be shown.
2. react/react-source-browser and react/react-source-server will be added.

[zetachang]

This has landed on master. Tutorials and hyper-react might need to be updated too, will check out before we are ready to make another release.

/cc @barriehadfield