Allow files in a pull request diff to be linked as "related files"

I'd like to have the ability, either when creating a pull request or reviewing a pull request, to mark two files in a diff as "related" so that they can be viewed side-by-side.

The use cases are when a file has changed type but is still related to another in a way (e.g. when a Java class file is converted to Kotlin), and when a file has moved directory and changed enough for Git to believe the file was removed+added elsewhere.