Add django_select2 to the global space.

When javascript output is wrapped django_select2 can not be found, adding it to the window instead of a var fixes this. There may be a better way. I hit this error with this feature of django-pipeline turned on (it is by default):
https://django-pipeline.readthedocs.org/en/latest/configuration.html#wrapped-javascript-output

Everything works under the test app with this change.