Check customized participation times for plausibility

[jeriox]

Consider an event that starts in the evening on the 01.01. and ends at 01:00 on the 02.01. As a planner, I want to customize the participation of a volunteer because they left at 22:00. When entering this time in the disposition form, the date part of the split datetime input is set to 02.01. If I don't remember to manually adjust this to 01.01, the volunteer will receive >24 working hours. It would be nice to check if the custom times exceed the duration of the event (maybe by some margin?) and display feedback to the planner.

[felixrindt]

I think this is related to #1060 because it could both be solved with better datetime inputs and display/validation.