batches: implement `createEmptyBatchChange` mutation

Created by: courier-new

Implements the mutation createEmptyBatchChange described in the empty batch spec RFC for initially creating a batch change from the SSBC UI. It takes namespace and name as required arguments, creates the batch change with an empty batch spec, and returns the new batch change's resolver.

I have one outstanding question I didn't address here but left TODO comments for, which is that, it's my understanding that we should enforce the combination of name + namespace should be unique for every batch change, but I'm not sure the best way to enforce this. I imagine we must already check this for batch specs executed from src-cli, but I couldn't find where that was happening.