Display unload warning if MR edit changed

What does this MR do?

This adds the functionality to display a warning if a user navigates away from the edit MR screen without saving changes

Issue to #290622 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• 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