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
Docstring please (here and L103)