refs
https://www.notion.so/ghost/Marketing-Milestone-email-campaigns-1d2c9dee3cfa4029863edb16092ad5c4
This adds a milestone entity and in-memory repository in a new
`milestone-emails` package. This also adds a first initial definition of
milestones and their types which is held in the default config to avoid
DB changes when, e. g. values change.
This should get everything in place to begin with the service
implementation.
