Implement ruby condition for milestones

Description

We could define how we add missing missed-deliverable as:

resource_rules:
  issues:
    rules:
      - name: Label ~"missed\-deliverable" for ~Deliverable
        conditions:
          state: opened
          ruby: milestone.nil? || (milestone&.succ && Date.today > milestone.succ.start_date)
          labels:
            - Deliverable
        actions:
          labels:
            - missed-deliverable

Local dry-run

Performing a dry run.

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Processing rules for issues

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Processing rule: Label missed-deliverable for Deliverable

[DEBUG] get_url: https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab-ce/issues?per_page=100&labels=Deliverable&state=opened ...

• Found 210 resources...
• Limiting resources....
• Total resource after limiting: 3 resources

The following comments would be posted for the rule Label missed-deliverable for Deliverable:

https://gitlab.com/gitlab-org/gitlab-ce/issues/47143

/label ~"missed-deliverable"

https://gitlab.com/gitlab-org/gitlab-ce/issues/44005

/label ~"missed-deliverable"

https://gitlab.com/gitlab-org/gitlab-ce/issues/35062

/label ~"missed-deliverable"

0.83user 0.10system 0:19.76elapsed 4%CPU (0avgtext+0avgdata 32876maxresident)k 0inputs+0outputs (0major+6631minor)pagefaults 0swaps

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Partially implements #62 (closed)

The other range labels: !44 (merged)