utilities/gitlab_recurring_issue#11: Top level label definition

Currently, we specify labels per ticket:

    tickets:
      - title: "Check Softener Salt"
        active: true
        schedule:
            nth:
                n: 3
                weekday: Sat
        description: "Check whether the water softener needs more salt adding"
        project: jira-projects/HOME
        assignee: btasker
        labels: ["Task","kitchen"]

However, I'd also like to be able to specify a set of labels that should be appended to any created ticket. In particular, I'd like to be able to add RecurringIssue so that it's easy to identify/locate tickets if this system ever goes nuts.