Feature/additional tests (part 2)

* adds tests for function assign_track_and_create_student, other controller methods, new model methods, and adds two more tests to cover all of experiment_pages
* note: this is Part 2 of feature/additional_tests.


There isn't currently. Instead, a HttpResponse is returned with an error message. We can change this to be an error raised. In any case, the UI will not allow this situation to happen.