refs https://github.com/TryGhost/Team/issues/1470
Instead of counting the MRR by resolving all the deltas from the past until now, we should start with the current calculated MRR and resolve it until the first event. That would give a more accurate recent MRR (in exchange for a less accurate MRR for older data) and allows us to limit the amount of returned days in the future.
- Includes MRR stats service that can fetch the current MRR per currency
- The service can return a history of the MRR for every day and currency
- New admin API endpoint /stats/mrr that returns the MRR history
- Includes tests for these new service and endpoint
