refs https://github.com/TryGhost/Ghost/issues/12602
* Added Event Repository
** Added method for MRR over time
** Added method for newsletter subscriptions over time
** Added method for gross volume over time
** Added method for status segment size over time
* Captured login events
* Captured newsletter subscription/unsubscription
* Captured email address change events
* Captured paid subscription events
* Captured payment events
* Captured status events
