Support for React 16.3 and Portal ?

[rockmandash]

[rockmandash]

It's too hard for me to refactor to React 16.3, because it is too magic

[berzniz]

What's the motivation for React 16.3? Is this because of deprecation warnings?

I would split the effort to:

1. Upgrading to React 16.3 and have it working as usual
2. Replacing the undocumented-portal with the official Portal component