de-nest modal/overlay elements

This solves issues with DOM event bubbling prevented by actions. bubbles=false hijacks events, so things like bootstrap widgets, whose events are handled on the document, wouldn't work inside the modal.

https://github.com/emberjs/website/issues/1081
https://github.com/emberjs/website/pull/1383
Merge pull request #1422 from denisnazarov/patch-2

de-nest modal/overlay elements in modal example