[Defer] Add modal logic to runtime

[zaydek]

Part of the logic needed to implement this is documented here: https://www.notion.so/Intro-to-React-2019-918bdd6954c1459e82caa748178f6455#3d4f31e5ba91410b99e66d8f2d0373ff.

The website itself should not be scrollable when modal logic is triggered.