refs TryGhost/Team#1458
refs TryGhost/Team#1459
refs TryGhost/Team#1372
- Added a new stats service, which is divided into several categories. Currently only the 'members' category for member related stats.
- When there are missing or corrupt members status events in the DB, the totals returned by the old member stats endpoint (`/members/stats/count`) were wrong. This is fixed in the new service by counting in reverse order and starting with the actual totals.
- New Stats API, with the new `/stats/members/count-history` endpoint.
- This new endpoint also returns the paid deltas -> dashboard 5.0 will show subscribed and canceled paid members for each day
- Includes tests for the new stats service and endpoint
