Update scrollbar functionality of .to-do

Ensure the scrollbar of the .to-do list is selectable for  Windows users. An explicit z-index was needed to ensure the scrollbar for the element using .to-do list was in fact selectable.

Used the main .to-do class to fix the scroll-bar functionality, but the `[data-progress="cleaning-house"] .to-do` selector may perhaps be the better place to put the z-index declaration.