Allow changing the speed of battle just like races

This MR adds a new menu option and CVAR kartbattlespeed that allows you to change the game speed in battle mode just like in race mode. The default for this new option is easy speed just like how it is in vanilla. This is a separate toggle from the race game speed, just in case a server owner wants to have different settings between race and battle mode. This was done as close to vanilla as possible and the CVAR and menu option respects unlocks just like it does for Race speed. The Discord RPC will also now show the game speed of battle mode as well.