WIP: Show viewing and viewed info in diff file tree

What does this MR do?

This addresses https://gitlab.com/gitlab-org/gitlab-ce/issues/52485, by making files in the diff file tree show bold when unread and highlight when they are currently focused.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

Closes #52485 (moved)