Add request argument to authenticate methods

[johnbradley]

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:

gcb-web-auth/gcb_web_auth/backends/dukeds.py

Line 38 in 51b74f2

def authenticate(self, token):

gcb-web-auth/gcb_web_auth/backends/oauth.py

Line 31 in 51b74f2

def authenticate(self, service=None, token_dict=None):

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