refs https://github.com/TryGhost/Team/issues/2400
- we've deemed it useful to start to return `Content-Version` for all
API requests, because it becomes useful to know which version of Ghost
a response has come from in logs
- this should also help us detect Admin<->Ghost API mismatches, which
was the cause of a bug recently (ref'd issue)
closes https://github.com/TryGhost/Team/issues/1942
- Added data fixtures for referrers
- Added new endpoint to fetch referrer stats for a given post: `/stats/referrers/posts/:id`
- Added new ReferrersStatsService, responsible for calculating referrer stats
refs https://github.com/TryGhost/Toolbox/issues/354
- this commit turns the Ghost repo into a monorepo so we can bring our
internal packages back in, which makes life easier when working on
Ghost