Tracking viewing of justifications

[profgrumpy]

Follow up on issue #34

The idea is to track the students who actually take a look (or not) at their peers' justifications. This may be used for the purpose explained below in the original post, or simply to validate whether this feature is even used by students beyond asking them if they like it in surveys.

Original post:

We might want to collapse all peers' justifications by default and allow students to either not look at them at all (we need to track this down in the model of the attempt) or allow them to consult them and apply likes.
This might allow to identify to what degree students are interested in peer instruction at all, and then characterize the performance profiles of students who are or are not