Add back update_developer_name command temporarily

(30 or so public apps have no developer_name set, they must have been
 had some issues when the command was ran, add it back so that we can
 fix them)
Merge pull request #1623 from diox/add-back-update_developer_name

Add back update_developer_name command temporarily
Create FeedApp model (+ docs, API endpoints) to feature individual apps in feed (bug 937338)
Ensure that SplitField output and input fields receive the parent field's context
API performance tweaks regarding apps (bug 956987)

- Re-use categories instead of using a new queryset
- Re-use all_previews cached list
- Don't load translations when loading versions
- Use a simplified serializer everywhere
- Check premium status before returning prices
Merge pull request #1528 from chuckharmston/937338-feed_featured_app

Create FeedApp model (+ docs, API endpoints) to feature individual apps in feed (bug 937338)
Fix migration number conflict.
Fixes tests broken by 8ee06d3
Merge pull request #1624 from diox/simple-apps-in-api-performance

API performance tweaks regarding apps (bug 956987)
has_premium() is a better check, it will check type first (bug 956987)