* [x] Replace the `date_alert` with `date_alert_start_date` and `date_alert_due_date` in the `Notification::REASONS`
* Begin of the insertion[x]End of the insertion Begin of the deletion [ ]End of the deletion Expose the Notification representer reason as `date_alert` in the representer.
* [x] Add additional information to `NotificationRepresenter` to be able to display:
* Finish date is in 2 days
* Start date is today
* Work package is overdue since 2 days
* [x] The information needs to be machine readable, not a fixed string
* That way, the notification card can display the correct difference between the current time and the start/finish date which will change between looking at the card on one day and the next one.
* Does not need to be implemented but the format should be extensible to later also display activity information like
* Assignee set
* Assignee unset
* Assignee changed
* Begin of the insertion[x]End of the insertion Begin of the deletion [ ]End of the deletion The information needs to reflect changes done to the work package after the notification is created.
* E.g. User is notified about the work package starting in 2 days. Another user moves the work package a week into the future. Then the notification card needs to say that it is starting in 9 days. It can also move the start date to the past.
* A due date notification can switch to an overdue notification when the due date is past.
* [x] Ensure the format works for other use cases, e.g.:
* Having a notification for the release notes of an OpenProject version update
* Having a notification for a downtime
* Having the assignee change (typically old value & new value & property) information as part of the notification
* Begin of the insertion[x]End of the insertion Begin of the deletion [ ]End of the deletion Handle not having an actor for a notification (needs to be changed in API documentation as well)
* [x] Adapt the API documentation