Edx-auth-backends

Two new commits that changed functionality:
* Add EdXOAuth2.auth_complete_signal on auth_complete()
* Store refresh_token in extra_data

The EdXOAuth2 backend will now pull the user_id from the JWT and
store it in the UserSocialAuth.extra_data field.

BREAKING CHANGE: The user_id scope is now required when using the
EdXOAuth2 backend for oAuth+SSO. This means that the oauth
application must first be configured to have access to the user_id
scope, which is not available by default.

Updates to the EdXOAuth2 backend:

* Supports the `_PUBLIC_URL_ROOT` social django setting.
* `logout_url()` allows `_LOGOUT_REDIRECT_URL` to be undefined.

Allow for logout redirect with EdXOAuth2 backend.

social_core consider email field protected and won't let it change. Added a pipeline function to update email address.