Resolve "Display non-markdown content in the WYSIWYG mode of the SSE::HTML"

What does this MR do?

It ensures that HTML within a markdown file doesn't get rendered as HTML in WYSIWYG mode, but instead plain text.

The base implementation wasn't robust as:

• <link> HTML (first uneditable block in the screenshots below) would get swallowed and result in source content change (deletion of the link entirely) that was not intended
• <div> and other HTML would still be interactive (though pointer-events: none; could alleviate this part) and rendered as HTML

Based on user feedback, in iteration we'll likely filter and selectively let certain HTML blocks render in the WYSIWYG mode. This is YAGNI for now though.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #221078 (closed)