Set start and end time for tutors

[m-decarli]

It would be helpful to be able to set start and end time for tutors.
It can look the exact same as for participants in the ./events/*/edit page under the tab "specification"

Example: winter concert
The Event starts at 20:00 and ends at 23:00
However, tutors have to help from 14:30-23:40.
With the calendar integration, the event is shown from 20:00-23:00.
It would be nice if in my calendar integration the time that I (as tutor) have to spend there is shown. This is better for planning