Added stripe config check to members site data
Added stripe config check to members site data

refs https://github.com/TryGhost/members.js/issues/43

- Adds new `isStripeConfigured` flag to public members site data which denotes if stripe setup is completed
- Helps clients like members.js/themes to configure payment behavior based on this flag

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