Make sure overlay prompts are dismissible using keyboard navigation

[aschweigert]

Reported by Calmatters: https://newspack-pub.slack.com/archives/G0122C8BVPC/p1649981164117259

Any prompt that is displayed over content (popups, toasters, etc) should be dismissible using the Esc key in addition to clicking on the X or outside of the body of the prompt.