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
- Remove fallback to authors for developer_name in Version (all
  manifests should have developer name set nowadays)
Merge pull request #1593 from diox/apps-and-collection-api-performance

Tweak Addon / Webapp transformers to improve performance (bug 956987)