Remove label creation from MonthlyPreparationService

This gives the MonthlyPreparationService a single responsibility: creating stable branches, so we'll rename and expand it in a future iteration.

Meanwhile, the label creation was better located in the PickIntoLabel class, which has now been expanded to handle the idempotency.

This is preparatory work for expanding MonthlyPreparationService into something like StableBranchService which both creates the branches, and then syncs them from Security to Canonical.