Remove the 'Category' model, use constants instead (bug 987880)

https://bugzilla.mozilla.org/show_bug.cgi?id=987880

**TODO:**
- [x] Cleanups in forms & validation (though devhub is working fine)
- [x] Fixes for existing tests
- [ ] Migration to convert existing data (for apps, collections, feed items)
```
>>> cats
['health-fitness', 'maps-navigation']
>>> len(json.dumps(cats))
37
```

Since space is cheap I'd prefer this to be too big. MySQL truncates by default, iirc, which would result in unparseable JSON, which is bad.