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
if the enable_notifications operation fails (for whatever reason) the page will throw an error at the user, but the user profile / registration will have been committed.  it would be better if this step were either added to the transaction inside the `with transaction.commit_on_success` block, after calling `_do_create_account`, or wrapped in its own `try` that logs a warning but continues in case of failure.