frontend: Alerter: use priority queue for alerts

Current behaviour

Alerts get queued up FIFO, and displayed at speaking speed (since #2280), so if a collection of alerts appear in close sequence then a critical alert at the end could waste several precious seconds while the alerter speaks every alert before it.

Expected or desired behaviour

If we instead use a priority queue for the alertStore.alerts, where priority is determined by the alert severity, the most important alerts would always appear soonest, slowed down only by the one which is currently being read.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.