Fix webIDEUrl for relative URL case

Issue: Incorrect Web IDE URL constructed when relative... (#477551 - closed)

What does this MR do and why?

This MR fixes a bug with webIDEUrl when the project path contains a string matching relative_url_root.

MR acceptance checklist

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Screenshots or screen recordings

No relative URL	With relative URL
no_relative_url.mov Regression testing: Web IDE URL is constructed correctly as expected.	relative_url.mov Web IDE URL is correct: although we encounter an error in the /oauth/authorize step, we no longer see the 404 error page. The authorize error is fixed in this MR: Fix relative_url_root Web IDE OAuth issues (!161328 - merged). Here's how it looks with the /oauth/authorize fix: relative_url_with_other_fixes.mov

How to set up and validate locally

Enable Web IDE OAuth flag

1. Make sure that web_ide_oauth feature flag is enabled by visiting /gitlab/rails/features

Set up relative_url_root locally in your GDK

1. Stop GDK with gdk stop
2. Add relative_url_root: "/gitlab" to your gdk.yml
3. Run gdk reconfigure
4. In resulting Procfile, add -authBackend http://localhost:8080/gitlab to the end of the line that starts with gitlab-workhorse: (this is an issue with gitlab-workhorse I discovered while testing. We'll fix the Procfile generation in a separate MR)
5. Start GDK with gdk start
6. Now you can visit the GDK at /gitlab
7. IMPORTANT: You might also need to disable vite, since that seems to have relative_url_root issues. https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/configuration.md#vite-settings

Related to #477551 (closed)