Fix Issue #2767: Positioning and closing issues with Reveal modal when "animation:'none'" option is set

In response to Issue #2767 

Fixes issue where modals with no animation or an animation of 'fade' appeared at the top of the document regardless of how far the user has scrolled down the page. The modal should instead appear at the top of the viewport.

Fixes issue where modals with no animation don't respond to attempts to close via the 'close-reveal-modal' button, the esc key, or clicking outside of the modal. Props to @RobertinoValue