Add new fields on issue create page

What does this MR do and why?

this MR resolves frontend part of https://jihulab.com/gitlab-cn/gitlab/-/issues/2709, and the branch is based on !1583 (merged)

Screenshots or screen recordings

QQ20230505-012119-HD

How to set up and validate locally

1. Change gitlab.yml

    development:
      jh_custom_labels:
        issue:
          - { scope: "priority",
              values: ["high","middle","low"],
              required: true,
              default: "middle" }
        epic:
          - { scope: "priority",
              values: ["high","middle","low"],
              required: true,
              default: "middle" }

2. Enable feature flag in rails console

   Feature.enable(:jh_custom_labels)

3. See config

   Gitlab::CurrentSettings.jh_custom_labels

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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