add featured image thumbnail to upcoming events to distinguish them

[tantek]

The upcoming events list (e.g. on https://indieweb.org/this-week/2017-11-24.html ) is looking a bit long and semi-undifferentiated which is making me wonder if we can do something to make the design more "interesting" - like including thumbnail feature-images for each even that has them, in a float left sorta style

e.g. https://indieweb.org/events/2017-11-30-homebrew-website-club has a u-featured image

[tantek]

If the event has a u-featured, use it, if not, look for a u-logo (e.g. like on https://indieweb.org/2017/Austin) and use that if present. This is only for upcoming events.