Add version of "tools of the trade" to usability

[dbosk]

The usability seminar need better connection to the course. We can achieve this by adding some prep to the prep of the usability seminar.

We can add a version of "Tools of the Trade". That would force them to realize the need to usability when using libraries.

We can also add a discussion of comparing Signal to PGP. The main difference in Signal's success compared to PGP is usability.