Printable coach reports

Added a "display options" dropdown that allows users to toggle on/off:
* A legend for what the colors mean
* Showing the # attempts (per exercise, per student)
* Showing the streak progress (per exercise, per student)

@wangguan59 @rtibbles if you have time to try/improve... please take a look.  

We've had a couple of requests for more informative and printable coach reports; this seemed like the easiest option (toggling things on)