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
@aboudreault Have a look at https://github.com/edx-solutions/edx-platform/tree/master/docs - I haven't found any existing doc for `ENABLE_DISCUSSION_HOME_PANEL`, but https://github.com/edx-solutions/edx-platform/blob/master/docs/en_us/internal/discussion.md seem to contain most of the current documentation about the discussion service and its LMS endpoints.