Adding download stats on subtitles.

This is less likely to show any usage, as this is only for lower versions, but nice to have.

Doing a PR for this because, in order to get stats on these, we must serve static files through Python.  This took some changes in `manage.py`, the best place I could find for setting app-specific defaults on management commands that we don't own.

@aronasorman you seem to be itching to close that issue, so could you take a look?

Once this is merged, I will test on the staging server before merging to `release-0.11` and pushing to the central server.