Item experiment

This branch does several things:

• Cleaned up item code
• Player count scales item frequency for all player counts, not just below 8. Bigger matches should rely less on item spam.
• Power item frequency in Battle Mode now scales with player count like Race, not including Karma players. Huge matches should start sorta tame due to the mass of players, but as it gets closer to the 1v1 it becomes even more hectic.
• Number of allowed invincibility items at one time scales with player count. Only approx. 1/4 of the server can be invulnerable at one time, instead of a static 2. This amount can never go below 1.
• Reduce frequency of Eggman items.

Before this can be un-WIP'd, the things labelled [UNTESTED] need testing in a 9+ player game, preferably as close to 16 as possible.