After SKARA-2588 is deployed, users are complaining about the behavior of auto labeler.

After discussion with Erik, we propose the following behavior for the auto labeler:
 1. The auto labeler will never remove labels that have already been added.
 2. When the auto labeler is trying to add a new label, it will try to upgrade this label to group label if applicable, but if the group label was manually removed by user, then the auto labeler will not try to upgrade it.
 3. Users can add any labels they want.
 Example:
 1. In the initial labeling, the auto labeler adds “hotspot-runtime” to the PR.
      (Current labels: “hotspot-runtime”)
 2. The user pushes a new commit that touch “hotspot-gc” and “hotspot-jfr”. The auto labeler upgrades these to the group label “hotspot” and adds it to the PR.
     (Current labels: “hotspot-runtime”, “hotspot”)
3. Later, the user pushes a new commit touches only “hotspot-gc”. The auto labeler will not add “hotspot-gc” to the PR since “hotspot” label is already there.
     (Current labels: “hotspot-runtime”, “hotspot”)