Cleaned member stats endpoints response
Cleaned member stats endpoints response

refs https://github.com/TryGhost/Team/issues/469

- Cleans up response format for mrr and volume stats endpoint to more consistent pattern
- Removes `unit` attribute for now as its not used

ErisDS/Ghost,TryGhost/Ghost,e10/Ghost,bitjson/Ghost,e10/Ghost,janvt/Ghost,bitjson/Ghost,TryGhost/Ghost,TryGhost/Ghost,janvt/Ghost,ErisDS/Ghost,ErisDS/Ghost