Add request argument to authenticate methods

Update the backends to accommodate a change in Django:

> The ``authenticate()`` method of authentication backends will require ``request`` as the first positional argument.

https://github.com/django/django/blob/27aa7035f57f0db30b6632e4274e18b430906799/docs/internals/deprecation.txt#L320-L321

Currently with the latest Django all gcb-web-auth backends are skipped due to this missing argument.

Our current `authenticate()` methods:

https://github.com/Duke-GCB/gcb-web-auth/blob/51b74f278a3234e1036cc111407ff2b951354873/gcb_web_auth/backends/dukeds.py#L38

https://github.com/Duke-GCB/gcb-web-auth/blob/51b74f278a3234e1036cc111407ff2b951354873/gcb_web_auth/backends/oauth.py#L31

Adding `request=None` as the first argument to `authenticate()` fixes this problem.