do not remove "API-Filter" from "Vary" header
update region flag icons
optimize content ratings in app resource (bug 948192)
condense region flag styles
Merge pull request #1590 from ngokevin/iarcoptimus

optimize content ratings in app resource (bug 948192)
fix api content rating handler test
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