End lab sessions early

Sometimes we want to stop students from enquing, e.g. if the queue is so full that handling all remaining requests will already go over the time of the lab. Updating the end time of the lab also means that the queue will be nuked 15 minutes after closing time, which is not desirable.

Suggestion: add a secondary field for "request end time" (advanced feature) separate from the "lab end time". No new requests should be able to be made after this time, but getting requests from the queue should be possible until lab end time + 15 minutes.