Improved price data access and setting for Tier
Improved price data access and setting for Tier

refs https://github.com/TryGhost/Team/issues/2078

We want to be able to easily get the price for a Tier based on the cadence for
use when creating Stripe Prices. We also want to be able to set the pricing data
to `null` for free Tiers, rather than erroring.

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