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.
Instead of having santad create a listener for SantaGUI to connect to
and then reverse the client-server relationship, have SantaGUI create an
anonymous listener that it sends to santad using the control interface.
Also add a queue for notifications so that blocks that occur while
SantaGUI isn't running will show up once it starts.
