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.