Add an endpoint to generate a combined image of foreground and background for events and workshops

[Pinpickle]

For events and workshops, we have an endpoint that spits out an image (jpeg or png?) that combines the foreground and background associated with it into one image. You should be able to specify different sizes for different social services such as Facebook and Eventbrite.

This would be useful to generate images for events on the fly, and not have to store precomposed versions or get designers to compose them when required. It would mean that images used are always up to date.

Would also be useful for og:image, following up in another issue.