Add ENABLE_DISCUSSION_EMAIL_DIGEST feature

This PR adds a simple feature to enable discussion emails digest by default for all users. On account creation, if 
 ENABLE_DISCUSSION_EMAIL_DIGEST is set to True in settings, the process will set the notification_pref accordingly.

Question: I wonder if the setting name shouldn't be more generic for notifications. Is that setting used for anything else than discussion digest?

 @jimabramson @antoviaque
This duplicates the code from https://github.com/edx/edx-platform/blob/92fa79dbd03b87427753bb3199fc253959aa8ce1/lms/djangoapps/notification_prefs/views.py#L106 -- it should be factored to a function/method in the notification_prefs app to avoid duplication, and exposing internal logic from that app.