Fix openapi viewer for relative url instances

Fixes #381047 (comment 1296098032)

What does this MR do and why?

This fixes the openapi viewer for instances configured with root-relative url.

This has been broken because the iframe sandbox loads on a separate origin which doesn't have access to gon.relative_url_root to load the webpack chunk properly.

Screenshots or screen recordings

Notice the gdk.test:3000/gitlab without /gitlab in the Network URL tab:

before	after

How to set up and validate locally

1. Configure a gdk instance by appending the following line to gdk.yml relative_url_root: /gitlab
2. Run gdk reconfigure
3. Create openapi.yaml, for example https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/api/openapi/openapi_v2.yaml

MR acceptance checklist

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