refetch event attendees on the client-side

[M0nica]

Currently the list of attendees for a given event is only determined at build time from the Gatsby Airtable Plugin.

The attendees should continue to be determined at build time with the Gatsby Airtable Plugin so that they are available as soon as the page loads without having to wait for JavaScript to load.

There should be some new client-side logic added to fetch the latest attendees from Airtable for a given event in a useEffect() and the event attendees always should be re-fetched after a form submission so that form submitters can see their attendee information reflected on the page.