drop django-qunit as we're not using it anymore

This PR aim is to spark discussion about what we want to do with
```django-qunit``` and js tests in general.

At the moment, the version of ```django-qunit``` we use isn't compatible with
django1.6 (I haven't check if a new version is available that fixes this), and
the tests we have been broken and unmaintained for a looong time now.

Also removed:
* ```media/js/zamboni/tests*```
* ```templates/qunit/```
* ```scripts/run_jstests.*```
* all occurences of ```jstestnet```