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.


I think you should rename this test to reflect that it's editing an Experiment from uniform to weighted.  You might have another test that goes in the reverse direction, and tests that go another way (i.e., CSV to uniform or weighted).