batches: add `state` to batch change resolver

Created by: courier-new

This isn't strictly necessary for EM2.4 but it was going to come up for EM3.1, it didn't require too much work, and it makes something slightly easier for me with the new batch change create/edit settings form. I don't have a strong opinion on isDraft boolean field vs. state field with enum state, but I went with the latter because it seemed like a plus to be able to match the BatchChangeConnection state filter and this field. 🙂