Allow collapsing files in pull-requests

In some cases, developers may review a pull-request that contains many files with many changes (in case of a big refactor or a new major feature).

In such cases, you may find yourself reviewing a PR which requires you to scroll around a huge page. (Let's not go into the best practice discussion that PRs should be small. Sadly, best practice are not always followed for various reasons).

It would be really helpful if there was an option to collapse all lines of a given file (for example, a file that has already been reviewed or a file that was auto-generate which we are not likely to review) - that way we won't need to scroll around a huge PR page. (keep in mind that files are ordered alphabetically in the PR and in no way in the logical order of what you're reviewing).