Turn ChangsetEvents.Labels into ComputeLabels function
Created by: mrnugget
I think it makes more sense to follow the same style for all of these "compute a state based on the changeset and its events" methods/functions. (*ChangesetEvents).Labels was the last one that we didn't convert.
Since I've been looking at it in #12198 I decided to change it to a ComputeLabels function (that also does the sorting)
This also changes the changesetResolver to always sort events and use the slice type.