Fix overflow:hidden for overlays (#260)
* Fix overflow:hidden for overlays
* Review comments
* don't use cssText, it has unknown support in IE11, plus this way doesn't have a bunch of strange semi-colons
fix overflow hidden not being removed during unmount as opposed to a closing of a dialog
add tests for all the changes
* missed a reference
* remove const and fix unnecessary async in tests
* add comment to help explain name
