Pop-up when new machine registers

This is something we discussed in NY. The concern raised was that it's easy to miss that a new machine registered. Right now an event is generated. The proposal is to show a pop-up. Some alternatives are possible, but it should be pretty obvious to a logged admin that new machine appeared. There are two aspects to consider:

1. Legitimate admin is trying to register his machine and should get feedback that the registration was successful.
2. An attacker is abusing registration API. The admin should be notified and be able to block the machine and also disable future registrations.

This is somewhat logical continuation of #1339.