Add number of commits in review process (prev/next buttons)

Description

At the moment when doing a review commit per commit, I have no idea how many commits I went by and how much do I need to review. It would be great for me to keep track and see how much I still need to review.

Example URLs

The URL does not show prev/next. But you can open a PR that has multiple commits, select one, and youll see prev / next buttons.

https://github.com/mavlink/mavlink-camera-manager/pull/594/changes/a02ae36205b66468223d521504ef6c44a443c597