Add DEP: JavaScript Tests

This is a rough draft for a DEP detailing a path to adding JavaScript unit tests to Django.

This proposal currently only details my recommendation for a test suite and a native JS testing process (without a Python wrapper), including my reasoning.

I have also included some open questions that still need to be addressed.

[Related django-developers mailing list thread](https://groups.google.com/forum/#!searchin/django-developers/javascript$20tests/django-developers/igkUkkdJLOM/i34wuLnKknEJ)