Add a data generator to replace landfill (bug 1061143)

Maybe use a dict instead, with categories as keys, and number of featured addons as value?

```
featured_by_cat = {}
...
if featured_by_cat.get(category, 0) < 5:
    generate_collections(addon)
    featured_by_cat[category] = featured_by_cat.get(category, 0) + 1
```