Merge pull request #1575 from diox/stop-storing-urls-in-es

Stop storing URLs of icons/previews in ES, generate them at runtime (bug 952153)
add GA to content ratings (bug 957333)
update pegi rating mapping after iarc update (bug 958458)
Merge pull request #1615 from ngokevin/iarcga

add GA to content ratings (bug 957333)
devhub tracking WIP
Fixed payment account in search api (bug 958604)
Added stats and revenue_stats perms to account api.
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