Collapsible sections or "Collapsibles"

We insert buttons via JavaScript for opening and closing collapsible sections. We use the data attribute data-component="collapsibles" as a hook for our JavaScript, as this keeps things nicely separated from classes, which could more easily be overwritten.

Buttons shouldn't use the pointer cursor, so we need to ensure the buttons have sufficient affordance by other means, such as background colour, border and/or box-shadow styles.