Resolve "You should not be able to claim a timeslot that already started"

This MR adds back a missing check to see that a student indeed is trying to take a time-slot that has not already passed. It also adds the possibility to circumvent this check in an advanced option. Additionally, while working on the advanced option, I also added an option to change the (currently hardcoded) pre-slot early-taking period for requests. This setting now defaults to 0.

Closes #146 (closed), #416 (closed)