Allow opt-out for "download batch row data as CSV" feature

Sometimes that just confuses the issue, e.g. when the batch provides a
download feature of its own.

tailbone/views/batch.py

@@ -65,6 +65,7 @@ class BatchMasterView(MasterView):
     """
     has_rows = True
     rows_deletable = True
+    rows_downloadable = True
     refreshable = True
 
     def __init__(self, request):
@@ -712,6 +713,7 @@ class BatchMasterView(MasterView):
                                        "Execute {}".format(model_title))
 
         # download rows as CSV
+        if cls.rows_downloadable:
             config.add_route('{}.csv'.format(route_prefix), '{}/{{uuid}}/csv'.format(url_prefix))
             config.add_view(cls, attr='csv', route_name='{}.csv'.format(route_prefix),
                             permission='{}.csv'.format(permission_prefix))