Replace moment.js with more modern date library

[ngottlieb]

Moment.js is no longer maintained and considered a "legacy project." We should replace it wherever we use it with a more modern date library. (This will resolve one npm audit vulnerability).

See here: https://momentjs.com/docs/#/-project-status/

[ngottlieb]

Note - we may not need a date library at all if we're just careful about our date-related code.