refs https://github.com/TryGhost/Product/issues/3648
- Refactored Members API RouterController.createCheckoutSession: Split the method into smaller parts so we can reuse individual parts for the upcoming donation checkout session.
- Wired up donation checkout creation
- Added donation events
- this switches us to using Nx for `ghost:dev`, which means we can
configure its dependencies and ensure that the TS projects are built
beforehand
- also switches to Nx for `ghost:archive` so we don't need to run
`build:ts` for all `yarn` commands
refs https://github.com/TryGhost/DevOps/issues/45
- this switches the monorepo over to using Nx instead of Lerna, because
we don't currently need the versioning+publishing capabilities
- this also adds an `nx.json`, which allows us to enable task caching
- also adds `build:ts` to the TS projects, which is cached for fast execution
- how these interact with the dev.js script will hopefully soon be
reworked to be a better experience
