Migrate to latest, split python-social-auth.

PSA was monolothic, now split, with new features, like
a DB-backed partial pipeline. FB OAuth2 version also upped.

Partial pipelines don't get cleared except when necessary.
They persist for special cases like change of browser while
still mid-pipeline (i.e. email validation step).

Refactor, cleanup, and update of a lot of small things as well.

PLEASE NOTE the new `social_auth_partial` table.

common/djangoapps/third_party_auth/api/tests/test_views.py

@@ -14,7 +14,7 @@ from openedx.core.lib.api.permissions import ApiKeyHeaderPermission
 from rest_framework.test import APITestCase
 from django.conf import settings
 from django.test.utils import override_settings
-from social.apps.django_app.default.models import UserSocialAuth
+from social_django.models import UserSocialAuth
 
 from student.tests.factories import UserFactory
 from third_party_auth.api.permissions import ThirdPartyAuthProviderApiPermission

common/djangoapps/third_party_auth/api/views.py

@@ -9,7 +9,7 @@ from rest_framework.generics import ListAPIView
 from rest_framework.response import Response
 from rest_framework.views import APIView
 from rest_framework_oauth.authentication import OAuth2Authentication
-from social.apps.django_app.default.models import UserSocialAuth
+from social_django.models import UserSocialAuth
 
 from openedx.core.lib.api.authentication import (
     OAuth2AuthenticationAllowInactiveUser,