This adds a user preference to view diffs file-by-file instead
of viewing all the diff files in one big list.
This helps with performance on large merge request,
but it can also be handy sometimes to view merge requests
file-by-file instead of in a full list of files.

The option to enable this feature is inside of user prefences.
For now this is also behind a feature flag so we can roll
this out safely.

https://gitlab.com/gitlab-org/gitlab/-/issues/222790