Fix Web IDE terminal button on relative URL root

What does this MR do and why?

This MR fixes #13833 (closed) where a URL being built on the FE neglected to check for gon.relative_url_root.

Screenshots or screen recordings

Description	Before	After
With relative_url_root: /gitlabbin
Without relative_url_root (should be no changes)

How to set up and validate locally

1. Add relative_url_root: /gitlabbin to your gdk.yml
2. Run gdk reconfigure
3. Start gdk with gdk start
4. Visit your gdk in the browser with the relative url root (example, https://gitlab.test:3443/gitlabbin)
5. Visit a project and click "Web IDE"
6. You should see the "Web Terminal" icon in the right sidebar and the request to ide_terminals/check_config will either be 200 or 422 (this is an expected error code).

MR acceptance checklist

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

References

Related to #13833 (closed)