refs https://github.com/TryGhost/Team/issues/2078
This removes the burden from the Tier object, and allows us to reuse
the existing slug generation implementation we have in our bookshelf
models.
refs https://github.com/TryGhost/Team/issues/2078
This pulls the current Tiers logic into its own package, the persistence part of
the work has not been done yet, that will be handled in core, so all bookshelf
model specific stuff is kept together.