Add explicit toggle for enabling/disabling notifications

[atc0005]

While the current behavior of automatically toggling notifications based on the presence of a webhook URL or SMTP server is useful, there may be other times where the sysadmin simply wants to explicitly shutoff a specific notification type, while still leaving the configuration setting intact (i.e., with the valid SMTP server or webhook URL in-place).

This would be particularly useful when demoing the application and/or during testing.