Having this default to `localhost:8000` upsets the people who use `127.0.0.1:8000` and the people who use a local domain like `airmozilla-local`. Perhaps we just make it empty by default and then something like this:

```
if not getattr(settings, 'SITE_URL', None) and settings.DEBUG:
    settings.SITE_URL = RequestSite(request).domain
```
The security doesn't matter when doing local development.