Fix for Twitter compatibility

[bradenmacdonald]

Description: When I implemented #8155 I only checked support for OAuth2. However some Open edX users are using login backends such as Twitter that are based on OAuth1.

This PR tweaks the new configuration system so it is compatible with both OAuth1 and OAuth2 providers.

I manually tested this fix and confirmed it works and allows Twitter login.

Sandbox: http://pr9313.sandbox.opencraft.com/login (you can test it out if you have a Twitter account)

Setup Instructions: https://groups.google.com/d/msg/openedx-ops/RerASgJQACM/UMFR_QfdHwAJ

Reviewers: @Kelketek and TBD

[antoviaque]

FYI @nedbat . Also @bradenmacdonald posted on the ML before signing off, but I don't see his email in the thread at https://groups.google.com/forum/#!topic/openedx-ops/RerASgJQACM (maybe stuck in the moderation queue?). Edit: It appeared in the meantime. Thanks if someone unstuck it : )

[nedbat]

Yeah, sorry, and I responded with a link before noticing that his message was in the moderation queue. Thanks for being so diligent, @bradenmacdonald!

[bradenmacdonald]

@Kelketek This is ready for your review.

[Kelketek]

👍

[bradenmacdonald]

@nedbat @cpennington Is either of you available to review this SSO PR? (If not I'll ask Sarina to schedule something. There's no deadline for this.)

[cpennington]

Looks like a nice simple change. 👍

[bradenmacdonald]

Rebased and squashed; merging now. Thanks for your help with this @nedbat, @Kelketek and @cpennington !