Fix loading @gitlab/web-ide with relative_url_root

What does this MR do and why?

This MR fixes an issue where the new GitLab Web IDE wasn't loading when GitLab is hosted with a relative_url_root.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Add a relative_url_root: "/gitlab" to your local gdk.example.yml.
2. Run gdk reconfigure.
3. Run gdk restart.
4. Enable vscode_web_ide feature flag.
5. Checkout this MR's branch and visit the Web IDE for any project.

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.