More exercises, more multi-level exercises

[michaellevy]

These lessons work best with lots of exercises -- shout-out and multiple choice questions frequently interspersed and a couple students-go-code ("Challenge") exercises in each lesson. We need more of everything, especially in the second half of the lessons (~after Data.frame manipulation).

In all challenge exercises, some students finish quickly and others take longer. To deal with this, each should have at least one "bonus" harder challenge to keep the faster students occupied while the others work, and ideally a second level "advanced" challenge too.