Add a new event type for "new devices"

[lunkwill42]

It was suggested that it is comforting that a bunch of portstate events are opened when a new router is added, as it serves somewhat as a verification that the router is now being successfully monitored by Zino.

If we fix this in #257, it has also been suggested that we add a new event type to signal that a "new device has been discovered/added". This could serve as confirmation, without adding an additional eleventy events for every router port that is down on the new device.

Originally posted by @lunkwill42 in #257 (comment)