randomize which practice user is used

This pull requests randomizes the list of potential guest users before checking them to see if they can be used upon a click of the guest login button. This way users see a variety of seeds of problems when they log in as guest multiple times, rather than usually just seeing practice1 seeds every time.

I tested with 9 guest login accounts, 30 guest login accounts, and 1 guest login account. I logged in as guest, logged out, and logged in again many times and the specific login id changed each time (except with only 1 guest). And also nothing is broken for courses with no guest login.