Slack integration should be able to update channel when urgency changes

We have a PagerDuty service set up to post to slack every time a high urgency incident is triggered (but not for low urgency incidents). We noticed that for for some high urgency incidents we were not seeing slack messages announcing the incidents, although we were seeing e.g. announcements they were resolved.

We tracked this down to incidents which started as low urgency, but became high urgency. This means the initial trigger event is not posted, and because there is no way to update slack when the urgency changes we never see the incident start.

For slack integrations which post when high urgency events are triggered, the integration should probably also post when the urgency changes from low to high.