Migrate `unstable_renderSubtreeIntoContainer` to ReactDOM.createPortal()

[cjies]

Switch from unstable_renderSubtreeIntoContainer() to unstable_createPortal() for compatibility with React Fiber.

Related informations:

• [Portal] Switch from unstable_renderSubtreeIntoContainer() to unstable_createPortal() for compatibility with React Fiber palantir/blueprint#1509
• facebook/react#10309

[cjies]

ReactDOM.createPortal() is official now :) facebook/react#10675

[cjies]

unstable_renderSubtreeIntoContainer do no work in enzyme v3... 😕

And portal components are not supported in enzyme yet. (enzymejs/enzyme#252)

[cjies]

enzyme-adpater-react-16 v1.1.0 starts to support React Portal.
enzymejs/enzyme@3ebc14a#diff-c8bcb273fce5646f5438ffc519373594

Planning to upgrade React, Jest and Enzyme.

[zhusee2]

Addressed in #188.