Add AI Actions dropdown to content editor

What does this MR do and why?

Show generate issue description in editor actions, per https://gitlab.com/gitlab-org/gitlab/-/issues/406791

Test with additions from !124378 (diffs)

Screenshots or screen recordings

How to set up and validate locally

1. Set up AI for generating descriptions
2. In rails console enable feature flags

   Feature.enable(:generate_description_ai)

3. Visit a public project, go to an issue
4. Click Edit
5. See Dropdown actions in the description editor editor

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.