associate user with social identity based on email address

We have a number of existing accounts in the legacy IAR database which only have an email address to uniquely identify them. Make use of Python Social Auth's ability to associate users by verified email to "grandfather in" existing users.

Once merged, put #16 (closed) back to ~"workflow::sprint ready".

iar_project/settings/base.py

@@ -101,6 +101,7 @@ SOCIAL_AUTH_PIPELINE = (
     'social_core.pipeline.social_auth.social_user',
     'social_core.pipeline.user.get_username',
     'social_core.pipeline.user.create_user',
+    'social_core.pipeline.social_auth.associate_by_email',
     'social_core.pipeline.social_auth.associate_user',
     'social_core.pipeline.social_auth.load_extra_data',
     'social_core.pipeline.user.user_details',