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

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.

app/assets/javascripts/behaviors/markdown/render_sandboxed_mermaid.js

@@ -72,11 +72,14 @@ function fixElementSource(el) {
 
 export function getSandboxFrameSrc() {
   const path = joinPaths(gon.relative_url_root || '', SANDBOX_FRAME_PATH);
-  if (!darkModeEnabled()) {
-    return path;
+  let absoluteUrl = relativePathToAbsolute(path, getBaseURL());
+  if (darkModeEnabled()) {
+    absoluteUrl = setUrlParams({ darkMode: darkModeEnabled() }, absoluteUrl);
   }
-  const absoluteUrl = relativePathToAbsolute(path, getBaseURL());
-  return setUrlParams({ darkMode: darkModeEnabled() }, absoluteUrl);
+  if (window.gon?.relative_url_root) {
+    absoluteUrl = setUrlParams({ relativeRootPath: window.gon.relative_url_root }, absoluteUrl);
+  }
+  return absoluteUrl;
 }
 
 function renderMermaidEl(el, source) {