Disable eternal editing rights from end-dateless events

[aleksisalonen]

Currently you can edit events without end dates ad infinitum in the UI, even thought the API does not allow editing past events.

For events without an end date, build a validation that checks whether the event as a whole is situated in the past and disallow editing past ones.

[Rikuoja]

In the backend, events without end times always end at midnight. Therefore, the same rule should be used here, i.e. if end time is not specified, event can be edited until midnight on starting day.