Apply Accepting merge requests to all scheduled issues

Every issue associated to the Backlog milestone should be labelled ~"Accepting merge requests" too

Requires:

• https://gitlab.com/gitlab-org/gitlab-triage/issues/47

• And a recurring nightly rule here in triage-ops to apply the label to new issues or issues added to a milestone that meet that condition:

      - name: Apply `Accepting merge requests` to all issues with a milestone 
        conditions:
          milestone:
            - Any milestone
          forbidden_labels:
            - Accepting merge requests
          state: opened
        actions:
          labels:
            - Accepting merge requests