Allow batch view to disable the Execute button in some cases.

Although this only disables the UI button element, it doesn't really
prevent anything beyond that...

tailbone/views/batch.py

@@ -473,6 +473,7 @@ class BatchCrud(BaseCrud):
             'batch_display': self.batch_display,
             'batch_display_plural': self.batch_display_plural,
             'execute_title': self.handler.get_execute_title(batch),
+            'execute_enabled': True,
             'route_prefix': self.route_prefix,
             'permission_prefix': self.permission_prefix,
             }