Allow users to undo/rollback a complete import batch

Allow users to undo/rollback a complete import batch.

## Implementation details

- Use `batchId` stored in `externalData.importDetails` to identify all transactions from an import
- Add endpoint: `DELETE /api/v1/import/batch/{batchId}` to delete all transactions with matching batchId
- Show confirmation with count of transactions to be deleted
- Recalculate account balances after deletion