Link sub-event date changes to super-events

[aleksisalonen]

If the date of an existing sub-event is changed, it should be reflected on the super-event, if that change affects the boundaries of the super-event.

So a cross-check should be performed for sub-event saving and changes made to the super-event when needed.

[Rikuoja]

The next question is, do we actually want to enforce this at the API level?

Currently, the backend and database doesn't care about the times of super and subevents. Are there use cases where we should allow subevents outside superevent times?

[aleksisalonen]

Maybe we should enforce it at the API, but only after we have the proper UI functions.