Add image display to individual and listed events

[amandafrench]

It's common now for event listings to have a featured image that can be shared on social media, e.g., https://datacite.org/event/improving-equity-and-inclusion-with-datacites-global-access-fund/ .

Once #111 is approved and we have individual event pages, we can start creating custom images for each event and including them in page metadata to make image-enriched social sharing of events easier. Once we do that, we can incorporate images into event pages and lists, defaulting to the ROR logo for events with no images.