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
please also add a note warning that, in a default install, setting this to True when ENABLE_DISCUSSION_HOME_PANEL is set to False is not recommended, since subscribers who receive digests in that case will only be able to unsubscribe via links embedded in their emails, and they will have no way to resubscribe.