Proposal: Updates workflow for incident review

This proposal along with gitlab-com/gl-infra/production!46 (merged) changes the way we schedule and require incident reviews. I'm hoping that this will help drive the process to be a bit more async and help to highlight and prioritize issues that need extra review without the overhead of automatically giving every S2 incident a review.

Summary of changes:

• Keeps the requirement that all S1 incidents require an incident review
• Removes the requirement that all S2 incidents require an incident review
• Adds a new label "Review-Requested" that gives us a bit more flexibility to request reviews for any incident.
• All incidents should have corrective actions, if there are no corrective actions then this is probably a good reason to request a review.
• If an incident has corrective actions and there is no review requested for an S2 or lower incident, it is ok to close it.

Why is this change being made?

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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