Treat non-scoped group label as a group label

Spinning off from #631 (comment 480174901)

Instead of treating non-scoped group label a special case, we can just treat them the same. This means whenever there's a non-scoped group label (e.g. ~"group:certify"), we do not need to infer another group label.

This way, when there's ~"group:certify" we can still infer categories for it, and also make it possible to infer that from the categories. This will also require gitlab-com/www-gitlab-com!71331 (closed) to specify which exact label we need to use for the group label.

With this we can fully resolve #631 (closed)

For now we're just working around it by silencing it when ~"group:certify" is added.