Disable scrolling of background content while modal is open
Disable scrolling of background content while modal is open

This commit disables vertical scrolling of the background content while
the modal is open. This covers scrolling while the cursor is positioned
off of the modal window and for smaller viewports, scrolling beyond the
modal content, which would also cause the background content to scroll.

xtina-starr/reaction,artsy/reaction-force,xtina-starr/reaction,artsy/reaction,xtina-starr/reaction,artsy/reaction,artsy/reaction-force,xtina-starr/reaction,artsy/reaction