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
Does this work? I guess it's a synchronous request? Does it block the UI while the request is in progress? If yes, then we probably want to refactor `getDataSources` to use a callback.