Make sure the send_welcome_email field is not required

Fix #327 (closed)

Since all Field subclasses have required=True by default, the validation condition here is important. If you want to include a boolean in your form that can be either True or False (e.g. a checked or unchecked checkbox), you must remember to pass in required=False when creating the BooleanField. (https://docs.djangoproject.com/en/1.10/ref/forms/fields/)