Incident severities and escalation rules, monitor downtime relationship

[cjanz4711]

It is not very clear how incident severity levels effect on-call duty escalations and monitor statuses.

Please expose this relationship in the escalation rules so that we can have different rules for different severities.

Not all severities should be counted as downtime. Let us determine if a monitor should be placed into "up", "degraded" or "down" status based on incident severity.