fix(overlay): ARIA attributes for buttons

[skirtles-code]

Closes #928.

Elements with aria-label must also have a suitable role. This problem is currently being reported by ARIA testing tools, including the built-in Lighthouse tool in Chrome.

The roles that support aria-label are listed at:

• https://www.w3.org/TR/wai-aria/#namefromauthor

I think role="button" is the most appropriate for this use case. I'm not sure why these buttons are using <div> rather than <button> but I haven't changed that.

I've made similar changes to the other button.

I've also added aria-pressed as both buttons function as toggle buttons with pressed and unpressed states.

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