Added benefits relation to Product model
Added benefits relation to Product model

refs https://github.com/TryGhost/Team/issues/806

This relation sets up the ability to both read and write relations via
the Product model, allowing us to expose benefits via the Admin Product
API.

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