refs https://github.com/TryGhost/Team/issues/958
- The module contains a service class and not an api index as index.js file should. This rename also fixes an ESLint warning around index.js file being too complicated.
- The serivice should ideally be extracted into the member repository in the future iteration
refs #12537
- Class syntax is preferred over module functions because of constructor parameter injection (DI) which allows for easier module decoupling and testing
refs #12537
- `stats` method in members controller is quite big and does much more then controller method code should - few calls to relevant modules
- Extracted code "as is" into members serivce
- Next step will be to refactor this module as a class pattern with DI parameters
