Resolve "Merge request e-mail link has full URL"

What does this MR do?

This ensures the "Reply to this email directly or view it on GitLab" link in notification's footer isn't escaped.

Are there points in the code the reviewer needs to double check?

The added test passes even without the addition of .html_safe but this fixes the bug in development.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #49899 (closed)