When users change their email address, their old email is still an existing username #100
Description
- User with name1@something.com makes an account.
- They change their email address to name2@something.com.
- They try to log in with name1@something.com. The website will be confused and tell them they don't have an account (because of the Salesforce integration) but it will also be confused when they try to register with this email address.