Filter/sort leaderboards by overall rank XP

[Arkanoid0]

In a discussion around group points, the idea was raised for a system to allow the player to find/view times of a similar skill level to the player, and this crystallized into the idea of a Rivals leaderboard, a leaderboard similar to the current "Around" leaderboard, but rather than times close to your PR, times close to your skill level (the exact implementation will need testing and tweaking, but current thinking is the times of people with a similar amount of Rank Points in the mode)

The thinking is that this system would provide the player with a benchmark of what they should expect to be able to achieve time-wise on any particular map, as well as the strats most likely to be useful at that skill level via the replay system.