Model iteration setup

I initially had more than this, but pared it down after a conversation about the first iteration of this feature.

My other work is here: https://github.com/dimagi/Vellum/compare/hq-data-api
Oops. I think that may be something I did when testing it. Should totally be a callback.

Not sure I'll be able to fix it in time for you to merge though. (offline this week, on a plane later this morning)