Security fixes

Only return timetable entries for the current session when updating a session through the timetable (#3474, thanks @glunardi for reporting)
Prevent session managers/coordinators from modifying certain timetable entries or scheduling contributions not assigned to their session
Restrict access to timetable entry details to users who are authorized to see them

Improvements

Point to correct day in "edit session timetable" link (#3419)
Fix error when exporting abstracts with review questions to JSON
Point the timetable to correct day in the session details
Fix massive performance issue on the material package page in big events
Fix error when using the checkin app to mark someone as checked in (#3473, thanks @femtobit)
Fix error when a session coordinator tries changing the color of a break using the color picker in the balloon's tooltip