Compute lightweight theme popularity and movers (bug 1059964)

Fixes [bug 1059964](https://bugzilla.mozilla.org/show_bug.cgi?id=1059964)

This PR is based on #252
More readable (to me):

```
three_weeks_avg = (ThemeUpdateCount.objects.values('addon_id')
                                   .filter(date__gt=start - timedelta(days=21))
                                   .annotate(Avg('count')))
```

Not sure of the line length though.