Add button to summarize issues

What does this MR do and why?

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/407887 Adds a new UI for the summarization of Issues and removes the existing dropdown

TODO

• Add tests
• Move code into ee/
• Likely some cleanup

Screenshots or screen recordings

Screen_Recording_2023-04-19_at_19.51.10

How to set up and validate locally

• Enable feature flags

  Feature.enable(:openai_experimentation)
  Feature.enable(:summarize_notes)
  Feature.enable(:summarize_comments)

• Set an openai_api_key in your ApplicationSettings.

• Visit an issue, and click on 'See Summary'. On a private project or a confidential issue on a public project, this is not visible.

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.