Add concept of event images

[leesto]

We probably need events to support images which we can provide to be used on the site. I can see a need for two images at the moment:

• A hero image would allow us to have a different hero image for the event
• An event image (possibly event thumbnail) would be useful on pages such as the list of events to break them up (I imagine this image being either a photo or even the title slide we tend to use)

Linked with this we also end up with a gallery of photos on meetup - integrating that feels like a different ticket though as it is slightly different to these.