Associate identities by email

This requires careful selection of auth providers, but it is what most
people want in Weblate.

weblate/settings_example.py

@@ -183,6 +183,7 @@ SOCIAL_AUTH_PIPELINE = (
     'social.pipeline.social_auth.social_details',
     'social.pipeline.social_auth.social_uid',
     'social.pipeline.social_auth.auth_allowed',
+    'social.pipeline.social_auth.associate_by_email',
     'social.pipeline.social_auth.social_user',
     'social.pipeline.user.get_username',
     'accounts.pipeline.require_email',