refs https://github.com/TryGhost/Team/issues/1083
We combine the duration and duration_in_months into a single value
object which can be validated together, meaning we will never have
properties which are out of sync (e.g. forever durations with 2 months).
refs https://github.com/TryGhost/Team/issues/1083
Instead of Offers being hardcoded to the "once" duration this will allow
Admins to start creating offers of variable durations.