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
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/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
As these are dates we have no need for the timestamp at the end of them,
formatting as ISO8601 works with the rest of our system correctly, for
example in the client where we populate missing dates
refs https://github.com/TryGhost/Team/issues/1466
We need to include an exhaustive list of tiers and cadences in the
metadata so that clients knows which data to expect, as we have multiple
rows for a single date, broken down by tier/cadence.
We also include the totals broken down by tier/cadence for use with the
paid mix chart.
