Add potion resolving

Adds functionality for #2. This only resolves Harm, poison, and wither potions, which either do damage over time, or are a one time potion. Harm potions are tracked through thrown potions, since it appears that they are never added to the player's active potion effects, while poison and wither potions are, and are tracked through EntityDamageEvent. If it's an instanceof a damage potion then the shooter will be the entity that threw the potion, otherwise the damager is null. If you want to track the actual damager over time for poison and wither potions, let me know.