Email change doesn't update Unique model

[ronw23]

The email change functionality does not delete/create Unique models for the changed email. The effect of this is that you can have multiple accounts per email address:

1. User1 signs up with user1@example.com and validates
2. User1 changes email to user2@example.com and validates
3. User2 comes along and is able to sign up with user2@example.com as email, though not user1@example.com