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

https://bugzilla.mozilla.org/show_bug.cgi?id=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.