Make isOverflowVisible on ModalDialog a required prop without a default value?

[brian-smith-tcril]

As of #2939, isOverflowVisible on ModalDialog defaults to true. This can lead to issues such as openedx/frontend-app-course-authoring#986.

If, however, isOverflowVisible is false - we encounter the original problem seen in #2938.

With the current implementation either value being set as the default will lead to some consumers encountering unintended behavior, so I'd like to propose we don't set a default and instead require consumers decide if they want overflow or not.

It's also possible there's a more robust solution I haven't thought of yet, and this may tie in to #2990 somehow.