Add a config option to enable/disable "missing email" notifications.

[briskt]

The notifier formerly was used only for "missing email" notifications, but is now used for "new user". Only one config variable was used to enable/disable the "missing email" notification: the NOTIFIER_EMAIL_TO variable. The code in main.php just replaced the notifier with NullNotifier when no NOTIFIER_EMAIL_TO was configured. Since we now need the notifier for two kinds of messages, we can't use NullNotifier so we need another way to block the "missing email" notification. Currently, the sendMissingEmailNotice function just returns early if no NOTIFIER_EMAIL_TO is configured, but this is not as clean as it could be. Better to add a ENABLE_MISSING_EMAIL_NOTIFICATION config option when a breaking change is warranted.