Rationalize our use of unsubscribe()

[NicMcPhee]

We don't appear to use this at all in Labs 2 and 3, but we do have it in Lab 4 and the iteration template. In reading the Internet on this, it feels like we might find it useful to use take(1) or first() in all this, but I'm not really sure what the best practices here are.

See this issue in the iteration template for more.

[kklamberty]

Upon further discussion, this probably doesn't apply to Lab 2 at all, so Nic moved this to the Lab 3 repo.

[kklamberty]

If we like what's present in this PR, we should be sure to also make the change here. I have the impression there may be more to this, but it's worth noting that it was looked at in the iteration template.

[kklamberty]

There is a PR and branch in Lab 4 that does some/most things related to this: UMM-CSci-3601/3601-lab4-mongo#68