The most important actors (and soon, the only actors) on meetup are chapters and individuals.

An event photo isn't a person, so it's an `avatar-chapter` (any avatar-sized photo that can represent or is associated with chapter activity is an `avatar-chapter`).

We're actually in the process of killing sponsors, but if we do need to introduce a new type of entity with an avatar, we can extend a square or circle as design feels appropriate to represent  that entity.