[Users] Change Email to Required Field for User Registration

[BethanyG]

Change email in users_user to be a required (non-blank) field.

Since we are using a custom user model, this change will need to be made in the model.py file of the users app.

• Custom User Models (Django Docs)
• Referencing the Custom User Model (Django Docs)

[BethanyG]

Closing as duplicate.