Fix missing 'user' object in installed apps API (bug 944419)

For InstalledView, We were re-using AppSerializer (yay) but
without passing the necessary info (doh) because it was inside
a method in AppViewSet. I removed the method and moved stuff
to the serializer to make it easier to re-use.

Bonus: also fixes price stuff, which depend on region.
Merge pull request #1536 from diox/fix-installed-api

Fix missing 'user' object in installed apps API (bug 944419)