This allows a sync server to send a `custom_url` field along with a rule blocking execution and this will be used as the URL for the "open" button in place of the normally generated URL.
* Add a USB device blocking popup.
* Refactor SNTNotificationManager and SNTMessageWindowController to make
for generalized notification logic
* Add the configuration keys for custom block messages and resize window
Sometimes the GUI isn't running. Sometimes the user is using SSH. Either way, printing a message to the TTY of the parent of the just denied process is user-friendly.