We're planning to change this from a warning to an error and need to
clean the codebase up before we do so.
In all of these cases the shadowing was known about and was not
causing unexpected behaviour, so the refactor consists entirely of
renaming, rather than refactoring/bug fixes.
refs https://github.com/TryGhost/Team/issues/1939
- changes empty referrer source to null instead of explicit `Unavailable` in API
- makes it easier for clients/frontend to ignore empty referrers
refs https://github.com/TryGhost/Team/issues/1673
- We don't want to show updated subscriptions in the dashboard
- We need to send add fields to accomplish this
- Includes updated tests
