Add setting for external redirect page for job artifacts

What does this MR do and why?

Add setting for external redirect page for job artifacts

Create a new migration in application_settings and add setting to API and helper

Modify controller to skip linking to external redirect page if setting is disabled (and add corresponding test)

Add administration documentation for the new setting

Changelog: changed

Fixes #384228 (closed)

Screenshots or screen recordings

With setting enabled (default):

With setting disabled:

How to set up and validate locally

1. Enable GitLab Pages in GDK
2. Create a new project, import from Pages plain HTML
3. Run pipeline
4. Browse job artifacts
5. Clicking index.html should open external redirect page (current behavior)
6. Open admin area
7. Settings
8. CI/CD
9. Expand Continuous Integration and Deployment
10. Scroll to bottom of section
11. Uncheck "Enable the external redirect warning page for job artifacts"
12. Save changes
13. Reload job artifacts browse page
14. Clicking index.html should now go straight to index.html without redirect page

MR acceptance checklist

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

Related to #384228 (closed)