Provide a management command to generate download URLs for message store data.

Sometimes when doing a bulk download, piecing together the URLs one by one is super tedious.
I recently discovered that a better pattern is to use ``call_command``. See e.g. https://github.com/praekelt/vumi-go/blob/883d8d6f62101cee137e12f1579a32bc2a820ee7/go/base/tests/test_go_system_stats.py#L25-L29