Add jh_custom_labels feature and config

What does this MR do and why?

Related to issue https://jihulab.com/gitlab-cn/gitlab/-/issues/2709

Add jh_custom_labels feature and config

frontend part MR !1607 (merged)

/cc @qk44077907

Screenshots or screen recordings

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How to set up and validate locally

1. Change gitlab.yml

    development:
      jh_custom_labels:
        issue:
          - { scope: "priority",
              required: true }

2. Enable feature flag in rails console

   Feature.enable(:jh_custom_labels)

3. See config

   Gitlab::CurrentSettings.jh_custom_labels

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.