Add ENABLE_DISCUSSION_EMAIL_DIGEST feature, to enable discussion emails digest…

Add ENABLE_DISCUSSION_EMAIL_DIGEST feature, to enable discussion emails digest by default for all users

common/djangoapps/student/tests/test_create_account.py

 "Tests for account creation"
 
 import ddt
-import unittest
 from django.contrib.auth.models import User
 from django.test.client import RequestFactory
 from django.conf import settings