no issue
- In preparation of using event emitting for Milestone achievements, we needed to add a dedicated `MilestoneCreatedEvent` to the `Milestone` entity.
- The event will be emitted using `DomainEvents` when a new milesteone is saved, which will allow us to listen to these events.
no issue
- The way we're going to implement milestones diverged from the original idea of handling email sending within the milestone-emails package, as we'll be sending events instead and will utilise the StaffService to listen to them and send the emails
- This renames the package as well as the service in core itself and all relevant tests
