add uffd oauth2 backend

[davidc]

Proposed changes

Adds a new backend to support uffd single sign on - https://git.cccv.de/uffd/uffd

As there are multiple uffd installations, the base URL is configured through config not code.

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Other information

[nijel]

Can you please add tests?

[davidc]

Can you please add tests?

I'm not really sure how I can do that since uffd instances are private

[nijel]

The testing doesn't happen against real service, the responses are mocked. It does mostly test processing of user data, see for example https://github.com/python-social-auth/social-core/blob/master/social_core/tests/backends/test_coinbase.py