Ability to mark files/lines reviewed

When opening a pull request, there can be dozens or hundreds of files. It would be nice to have a way to track which files have been reviewed (tracking at the level of lines of code might be overkill), for instance by adding a checkbox/slider beside each file.

As we can "approve" individual commits, it would be nice to have the same there.

I am aware of the "Task" feature (which I love), but I cannot create a task for every file when a pull request/commit has 20+ files.