batches: support `replaceBatchSpecInput` with existing draft batch change

Created by: courier-new

Supersedes #28827 (and to a lesser extent, #28825, until we resolve what to do for executing a new batch spec on a non-draft batch change).

Updates replaceBatchSpecInput to re-associate the previous batch spec's batch change with the new one that is replacing it, but only if the batch change was still a draft. It will produce an error if you attempt to call it with a batch spec that was already applied to a batch change.

To do so, I stole BatchChange.IsDraft that Thorsten wrote from #28840.