Pre Django 1.10 the `username` field on the Django auth_user table is very small (something like 30-32 characters), so we've had to abuse the non-unique `email` field instead. Django 1.10's `username` field is 150 characters: https://github.com/django/django/blob/1.10.4/django/contrib/auth/models.py#L311 ...at which point we can stop abusing the `email` field.

This is being fixed as part of bug 1319246.