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
All units tests test the user creation with different parameters (language, header etc.). So that's why that call is repeated and not in setUp. It's fine to let things as they are imo.