Indicate length of created lab

Summary

When setting the times for a lab session, it would be great if the system could state the total length of the lab that was created. Possibly like so:

Reasoning

This would give direct feedback to the user, enabling them a sanity check on whether they set the lab times correctly.

Related to issue #727 (closed), where I described the experience of a novice lab user, accidentally setting up long overnight lab sessions.

In the screenshot idea I also added the grace period time, as that gives a bonus of directly explaining that the grace period is outside of the lab time and not inside 😄🙂