fix(Modal): Only set position when modal is open

[jeffcarbs]

Fixes #712

The modal has code that continuously calculates its position to:

• ensure it's centered vertically.
• add the scrolling class to the body if need be.

This was being called on component mount, which doesn't make sense since the modal itself may not actually be visible. This updates the the setPosition to start when the Portal gets mounted (the modal becomes visible) and stop when the Portal unmounts (the modal is hidden).

This should:

• reduce the workload being done by modals rendered onto a page.
• make it so the component doesn't touch the mountNode until the page has been fully rendered. This (I think) should fix Modal relies on document as default renderNode, breaks SSR #712 since the mountNode defaults to document.body which isn't present server-side.

[levithomason]

Awesome, nice catch!