Allow editing of SLO Result to indicate backout windows or rejected defects

[ricegi]

Provide a mechanism to take a given SLO result and post-edit it to annotate defects or alert periods that were part of expected blackout or items that should not apply to the SLO calculation.

In sure these items are well documented and the revised SLO calculations appear with suitable annotation.

[priestpl]

We have a use case where our application depends on a database, and if we have to take an application outage for 2 hours or more for database changes then we don't want this to count against us.