Draft: Allow high-level CPU players to select custom followers/colors.

This MR implements the ability for mid- and high-tier bots to select custom followers and colors at the start of a grand prix. This helps to characterize the bots more beyond the one rival and to reward players for unlocking new followers and colors, even if they don't recognize or like some of them enough to use them themselves (or maybe they have a favorite, and don't want to change it).

The current implementation allows followers to appear only with bots at or above DIFFICULTBOT level, meaning they don't appear in Relaxed mode and will appear with half the bots in Intense level. I do need some advice on how to implement follower/skin color changing, the latter of which would be reserved for Master/near-master level bots.

I also did a refactor on K_InitGrandPrix (but it's not very well split across commits, unfortunately), improving the readability; Please let me know how I did on that front.