Make certain exercises "Bonus"

[zenalapp]

Add "Bonus" to exercises that might be too much for the workshop, but good to keep on website. Ones I skipped:

• [x] Why learn to program (we kind of did this in the intro).
• [x] Why not use RStudio.
• [x] Running parts of our code (I did this with them instead).
• [x] Bad names for objects.
• [x] More line plots - just no time, but good exercise I think so maybe keep as not bonus.
• [x] Cheat sheet to find another univariate plot.
• [x] Ordering ggplot layers (did this one together).
• [x] Skipped transparency because talked about it when flipped geom_violin() and geom_point().
• [x] Maybe have only one save plot exercise? Although not bad to have practice...