Migrate beatmap carousel item sorting from List<T>.Sort()
to IEnumerable<T>.OrderBy(), as the second variant is documented to be
a stable sorting algorithm. This allows for eliminating unnecessary
movement of carousel items occurring whenever any set of items is tied
when changing sorting criteria.
Add an exemption clause allowing potential code contributors to submit
issues if they state they would like to work on them, and note that
mobile-related pull requests are still accepted.
Suggested-by: Dean Herbert <pe@ppy.sh>