Resrict slot claining once the slot has started

Summary

In slotted labs it would be useful if you can restrict the students from enqueuing once a concrete slot has started. Currently students can enqueue even if the slot has already started.

Reasoning

If multiple students claim a place in a slot that has already started this could create delays for the rest of the lab. Depending on the slot duration, enqueuing a few minutes later might not be a problem but if they do so 5 minutes before the end of a 20 minutes slot delays could become substantial.