keyboard navigation needs an accessibility review/overhaul

[Alex-Jordan]

Following a post about using ESC to exit the search tool, @ascholerChemeketa recommends a larger project to review/overhaul keyboard navigation.

See https://groups.google.com/g/pretext-dev/c/xSsSIeg-7fw/m/I_iDMfo_BwAJ.

[ascholerChemeketa]

Suggested goals:

• Make sure the core UI conforms to: https://webaim.org/techniques/keyboard/
• Look for places to reduce redundancy. I think there are a few different implementations of modals currently in the JS (calculator, search).