React-modal: Sprint for react-modal v4.

[diasbruno]

It's time for react-modal to get ready for react 16.3+, this includes:

Checklist

1. Deprecate old lifecycle methods and remove polyfills.
2. Decide how long we can maintain react-modal versions below v4 (backports or another approach).
3. Improve documentation where missing.
4. Add more examples.

If you want to give it a try on any item on the list, create a new issue and assign to you.

Appreciate ideas regarding this update and help with all the listed items.

[diasbruno]

653, #654 and #656 are good issues to be included in this sprint.

[kaushik94]

@diasbruno I am fixing #656 currently. I can help out with the other too also.

[wszgxa]

PR to fix #653 : https://github.com/reactjs/react-modal/pull/683

[diasbruno]

Thanks, @wszgxa. I'll have a look on the PR.

[diasbruno]

Moved to milestone 4.0.