Fix bug regarding broken mermaid markdown rendering in milestone view

What does this MR do and why?

This MR addresses the following issues / bug reports:

• Mermaid Graphs in Milestones (#389913 - closed)
• Mermaid Gantt Chart does not render in Milestone (#393049 - closed)

🛠 with ❤ at Siemens

Screenshots or screen recordings

The left side shows gitlab.com and the broken mermaid markdown rendering. Left side shows my local dev environment.

How to set up and validate locally

1. Go to any milestone detail page, e.g. http://127.0.0.1:3000/jashkenas/Underscore/-/milestones/6
2. Edit the milestone to include a mermaid markdown, e.g.

```mermaid
gantt
    title A Gantt Diagram
    dateFormat  YYYY-MM-DD
    section Section
    A task           :a1, 2014-01-01, 30d
    Another task     :after a1  , 20d
    section Another
    Task in sec      :2014-01-12  , 12d
    another task      : 24d
```

3. Save and the mermaid template should be rendered properly as shown in the screenshot

TODO

• Write feature spec in spec/features/markdown/sandboxed_mermaid_spec.rb
• Refactoring of new js code => align with existing codebase

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.

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• [-] Conforms to the database guides

Related to #389913 (closed) and #393049 (closed)