Publishability triggers preliminary alert

Trigger a preliminary alert for a superevent upon the first time that the preferred event is set to an event that meets the public alert criterion. Remove the preliminary alert timeout because it is no longer necessary to wait until the preferred event stabilizes.

This fixes a longstanding issue that has prevented automated preliminary alerts from being sent so far. The preferred event at the instant that the timeout ended did not meet the public alert criterion, but a preferred event that was selected some tens of seconds later did.

Note: merge !495 (closed) first.