Do a bunch of major modifications to the voting system's rule change system!

* Add `kartvoterulechanges`, or "Voting Rule Changes" on the menu, to allow some user control over event frequency.
	* "Never" - does what it says on the tin.
	* "Sometimes" - 1/8 chance of Encore if host has unlocked it, only gametype change when buffer is full
	* "Frequent" - 1/2 chance of Encore if host has unlocked it, gametype change every 5 maps
	* "Always" - If host has unlocked Encore, alternates between Encore and Gametype. Otherwise, always a gametype change
	* There's probably room for a setting between "Sometimes" and "Frequent", but I didn't want to overload the host with options and these were the ones that made sense.
* Better handling of buffer refreshes, to prevent two of the same map appearing next to each other in the voting unless there literally aren't that many maps.
* Mostly unrelated: Minor bugfix for Y_SetupVoteFinish, preventing music changes/random sounds playing on the You Will Join Next Race screen. (Branch-appropriate, at least.)