[fixed] Make the modal portal render into body again

[claydiffrient]

In c13fed9 behavior was changed
so that the modal portal ended up rendering into the app element.

In the default case where the appElement receives aria-hidden
when the modal is open, this meant that the modal itself was
also contained inside the aria-hidden element. This limits the
ability for screenreaders to read the modal contents.

Server-side rendering should not be affected by this change
because componentDidMount is only fired on the client-side thus
having a reference to document.body there should be acceptable.

Changes proposed:

• Make the portal append to body rather than the app element

Upgrade Path (for changed or removed APIs):

• Make sure that you are setting an app element (rather than using the default document.body) so that modal contents are not being aria-hidden

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[claydiffrient]

@diasbruno Would you mind looking this over and giving your thoughts before I merge and cut a new version?

[diasbruno]

Sure!

[diasbruno]

It's looks good. Good to go!

[ricardocosta]

Hello,

README.md should be updated with some urgency, because it's not clear how (or if possible) to have the modal rendered inside a specified DOM element.

Thanks!