Allow Portal to be used as top-level modal (#4338)

* Portal

* Allow Portal to be used in as both top-level and popover

* modal/popover variable naming

* export Portal in ~/ui

* Properly handle optional onKeyDown

* Add simple Playground Example

js/src/ui/Form/AddressSelect/addressSelect.js

@@ -176,6 +176,7 @@ class AddressSelect extends Component {
     return (
       <Portal
         className={ styles.inputContainer }
+        isChildModal
         onClose={ this.handleClose }
         onKeyDown={ this.handleKeyDown }
         open={ expanded }