API performance tweaks regarding apps (bug 956987)

- Re-use categories instead of using a new queryset
- Don't load translations when loading versions
- Use a simplified serializer everywhere
Tweak Addon / Webapp transformers to improve performance (bug 956987)

- Don't load categories, listed_authors in Webapp
- Don't load translations for previews in Webapp
- Don't call version transformer twice
- Do set _premium = None on addons that have no AddonPremium