Fix HTML escaping in StringRangeMarker

What does this MR do?

In Rails 6, html_safe is now taken into consideration when using the []= operator to replace text.

We need to mark the replacement text as HTML-safe so that it doesn't get escaped when it gets inserted into the HTML-safe string.

Example:

string = "<p>Hello</p>".html_safe
string[3..4] = "<b>He</b>"

"<p>&lt;b&gt;He&lt;/b&gt;llo</p>"

Correct way:

string = "<p>Hello</p>".html_safe
string[3..4] = "<b>He</b>".html_safe

"<p><b>He</b>llo</p>"

This was extracted from !19891 (merged)

Screenshots

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

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

#31034 (closed)