Reopen issues on a new note from external participants

What does this MR do and why?

Solves Reopen Service Desk issues when an external par... (#8549 - closed)

You can set to automatically reopen an issue when it receives a new note from an external participant by email on the project's Service Desk settings page.

The backend logic and database field was added in Reopen an issue on a new note from external par... (!137540 - merged)

This feature is not hidden behind a feature flag 🎏 and will be available directly to GitLab SaaS customers. This is why it also contains the documentation change.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. If you haven't set up incoming_email or service_desk_email for email ingestion, please add this to your gitlab.yml file in the development: section. Please restart GDK with gdk restart:

   incoming_email:
     enabled: true
     address: "incoming+%{key}@example.com"

   This will allow you to see the Service Desk section in the settings and generate project-specific email addresses. You won't be able to ingest emails, but we don't need that here.
2. Visit http://127.0.0.1:3000/flightjs/Flight/edit and expand the Service Desk section and find the Reopen issues on a new note from an external participant checkbox at the bottom of the settings form and check the checkbox.
3. Select Save changes. You should see a success alert. The checkbox should stay checked.
4. Feel free to check the request and response in the dev tools. Both the request and the response should contain add_external_participants_from_cc in the payload.
5. Reload the settings page. The checkbox should still be checked.
6. Uncheck the checkbox and select Save changes. The checkbox should stay unchecked.

Full feature test

See the validation steps in the MR that introduced the database field and logic. You don't need to run step 6 if you already checked the checkbox via the UI.

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.