Make relation between batch spec and batch change explicit in server-side execution

Created by: eseliger

For server-side execution, we shifted around how batch specs and batch changes relate to each other. Before: Batch spec has name and namespace fields, batch change as well, we match the two together on apply/preview. When the batch change doesn't exist yet, it is created on apply.

Now: The batch change is always created first. The batch spec has the name auto populated in the editor. We should make this relationship explicit by adding a batch_change_id column to the batch spec, using that in ApplyBatchSpec, and sending the batch change ID along with the content for createBatchSpecFromRaw.