feat(Modal): added support for React Portal

[fantua]

• Added react-portal package (uses React v16 and its official API for creating portals; has a fallback for React v15).
• Updated enzyme-adapter-react-16 to latest version for Portals support.

1. Currently handleBackdropClick is fires for nested modals and close all opened. Need to investigate more on this.
2. Most test is failing by:

expect(wrapper.childAt(0).children().length).toBe(0);

After using ReactDOM.createPortal it's not more 0. Can I remove this checks?

Need some reviews/comments to current changes.

Refers to #601, #761.

[fantua]

cc @TheSharpieOne @supergibbs

[supergibbs]

I haven't had a chance to mess around with portals yet but would it be easier to just wrap a Modal in another component in your app and tell it to render to a portal? Since reactstrap is still supports react 15 not sure if it should have react 16 stuff built in.

[fantua]

I haven't had a chance to mess around with portals yet but would it be easier to just wrap a Modal in another component in your app and tell it to render to a portal?

It's doesn't make sense because currently Modal uses unstable_renderSubtreeIntoContainer. renderSubtreeIntoContainer isn't stable and have some bugs (ex. #601), my changes allow you to:

• use Portal for 16 (which is stable)
• fix component logic and structure (currently it's looks a little bit weird)
• leaves fallback for 15

[seberik]

Calling destroy in componenDidUpdate breaks the close transition.

[TheSharpieOne]

Exit animations are not working
Closing a nested model closes all of the modals. (not even clicking the backdrop)
For the tests, you can change them however you need to; just ensure they have good/complete coverage and are actually testing (not just running through the code to get coverage) and pass.

[fantua]

Added new changes:

• fixed Modal exit animation,
• fixed nested Modals,
• updated react-transition-group to 2.2.1

@seberik @TheSharpieOne, guys take a look at this, looks like now all should be good 😃

[seberik]

The exit animation now works but when double-clicking the backdrop or escape-key it will close and re-open the modal.

[fantua]

Fixed backdrop double-clicking and tests.

[fantua]

Merged with master. Waiting for review.

cc @TheSharpieOne

[TheSharpieOne]

LGTM, just a small nit.

[TheSharpieOne]

You can remove all of the checks like this. The previous test was ensuring nothing was inserted into the DOM, but now using react-portal, we will always be inserting react-portal, so these checks are more/less pointless (unless you want to check that react-portal get inserted, but I would also only do that in one test, not all of them.)

[fantua]

Removed those unneeded checks and created new test for this.