[Queue-2.0] Lab Option for end-of-lab grace period

At the end of a lab, there is a grace-period of 15 minutes during which students can no longer enqueue, but TAs can still work on questions. We should have some more configurability for this. We should add an option that sets the duration for the grace-period.

Optionally, we could expand this by having an option for turning off the grace-period entirely or until all requests are handled.