refs https://github.com/TryGhost/Ghost/pull/12541
- make `EventProcessor` a super-class designed to be inherited from in consumer applications for application-level implementation
- helps to keep application-level concerns for event handling (eg, what to do with spam complaints) and things like application database knowledge in the consumer
- removed all database knowledge from `EmailAnalyticsService`
- requires a `queries` option to be passed in that lets the consuming application provide knowledge and define how fetched stats should be aggregated
