Reset webpack path for Mermaid iFrames

Fixes #413596 (closed)

What does this MR do and why?

This MR sets correct webpack path for Mermaid iFrames to render it correctly. This was broken after this upgrade: !115885 (merged) possibly because it tries to load more chunks.

The sandbox iFrame 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

Before	After

How to set up and validate locally

1. Spin up a GDK with a relative URL.
2. Create an issue and adds a mermaid diagram to its description or comments.
3. The diagram should now be generated correctly.

Example:

graph LR
  A(GitLab Rails) -- Writes new pages deployment --> B[(NFS)]
  C(GitLab Pages) -. Reads static content .-> B

MR acceptance checklist

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