refs https://github.com/TryGhost/Team/issues/2400
- we've deemed it useful to start to return `Content-Version` for all
API requests, because it becomes useful to know which version of Ghost
a response has come from in logs
- this should also help us detect Admin<->Ghost API mismatches, which
was the cause of a bug recently (ref'd issue)
refs https://github.com/TryGhost/Team/issues/1726
- free trial offers don't need a stripe coupon created for them
- checkout sessions for free trial offers ignore stripe coupon and directly pass the trial days value
- trial days of an offer take precedence over trial days added as default to a tier
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
