Prevent users from using existing emails to register

[TimOsahenru]

The User model of the authentication framework does not prevent creating users with the same email address.

If two or more user accounts share the same email address, we won’t be able to discern which user is authenticating. Now that users can log in using their email address, we have to prevent users from registering with an existing email address.

We will now change the user registration form, to prevent multiple users from registering with the same email address.

Deliverables

• Changes should be made from forms.py