Add system note for paging status changes

Motivation

Users responding to incidents need to understand who has been notified of alerts, when, and why. Having system notes allows responders to get a better picture of the history of an incident.

Scope

• Add a system note when paging on a policy begins
• Add a system note when the paging status changes
• Add a system note to the incident when a corresponding alert's status changes

UX

Detailed breakdown of what the notes should be

1. The status on alerts is referred to as 'status'
2. The status on incidents is referred to as 'incident status'
3. If the source of the initial change is a user, the user is credited across all objects
4. If the source of the initial change is automation, the user is alert-bot
5. If the change is being propagated from a different object, an explanation is appended to the original text of the system note
6. GFM syntax is relied on for referencing objects

Action	Object	Text
User changes alert status	Alert	[existing, unchanged] @user changed the status to **Acknowledged**
User changes alert status	Incident	[new] @user changed the incident status to **Acknowledged** by changing the status of ^alert#540
User changes incident status	Incident	[new] @user changed the incident status to **Acknowledged**
User changes incident status	Alert	[new] @user changed the status to **Acknowledged** by changing the incident status of #1176
User closes incident	Incident	[existing, unchanged] @user closed [existing, modified] @user changed the incident status to **Resolved** by closing the incident (add 'incident' prefix to status)
User closes incident	Alert	[existing, modified] @user changed the status to **Resolved** by closing incident #2536 (closed) (swap to 'incident' instead of 'issue))
Recovery notification resolves alert	Alert	[existing, unchanged] @alert-bot logged a recovery alert from **My test integration** [existing, unchanged]@alert-bot changed the status to **Resolved**
Recovery notification resolves alert	Incident	[existing, unchanged]@alert-bot changed the status to **Resolved** by closing the incident [existing, modified] @alert-bot automatically closed this incident because the alert resolved (remove extra period)