Support for non-recoverable alerts

Summary

Some alerts are "one-off", i.e. they inform about a singular event (not a continuous state), and have no recovery conditions.

Example: server reboot is an event which should trigger an alert, but there should be no notification about the return to the "correct" state.

So we want to support this simple fact in our Rocket.Chat notifications through the AlertNode.noRecoveries() method.