User is associated by email even though `associate_by_email` pipeline is disabled

[vanya909]

Ways to reproduce

1. Create django model with USERNAME_FIELD set as email
2. Create user with some email
3. Try to log in via social auth under the user with the same email

Expected behaviour

AuthForbidden should be raised, because originally this user was not created via social auth

Actual behaviour

User is being logged in

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Looks like if username field for the model is email then user will be logged in by this email even though it may be unsafe as it was mentioned in associate_by_email pipeline's docs

Is this an expected behavior?

https://github.com/python-social-auth/social-app-django/blob/master/social_django/storage.py#L91

[nijel]

#803 is my attempt to address this, review is welcome.