refs https://github.com/TryGhost/Team/issues/1875
- due to an misbehavior in our model layer, when `tiers` is set on a Post, it'll
trigger a save of the Tier, and this produces an extra event in the
`actions` table
- mapping the Tier(s) to just the ID prevents bookshelf-relations from
editing the Tier and thus prevents the extra event
- also fixed tests which were implicitly assuming supplying a slug to a
post would create the product
refs https://github.com/TryGhost/Toolbox/issues/354
- this commit turns the Ghost repo into a monorepo so we can bring our
internal packages back in, which makes life easier when working on
Ghost
