Add support for running javascript tests

[jarshwah]

The tox.ini provides a javascript environment, but the test environment relies on an old version of NPM: https://code.djangoproject.com/ticket/25803

This should probably be fixed upstream in some way. The grunt qunit test runner looks abandoned, so it might be necessary to look into something else. Once that is done, we can properly alias a command to run the javascript tests, just as we've done for docs/isort/flake8.