Unify summarize comments checks and improve errors

What does this MR do and why?

These changes

• Use a policy to check for the summarize_notes feature so it can be re-used for the backend and frontend
• Shows errors coming from the API

Screenshots or screen recordings

Screen_Recording_2023-04-18_at_13.45.03

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 scroll down to write a comment. Note the new AI Actions dropdown in the markdown editor toolbar. 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.