[DONE] Weapon mod rework

This is a rework of the current weapon mod system. The current limit of 4 mods per weapon has been removed in favor of this new system.

Adds three new weapon mod_targets: bow, crossbow, and launcher.

It adds the ability for each weapon to specify valid mod 'locations' and the amount of each. 
( JSON example: "valid_mod_locations": [[ "accessories", 2 ], [ "grip", 1 ]] means the weapon has two accessory mod locations and one grip mod location. )

Each weapon mod must specify it's mod location. ( JSON: "location": "accessories" means the mod can be installed on any weapon with one available accessory mod location. )

This also makes the pistol crossbow only accept mods that can be installed on both crossbows and pistols. This is to prevent one from attaching, say, a masterkey to the bottom of it.

Current locations:

accessories 
barrel
bore
conversion
grip
magazine
mechanism
muzzle
rail
sights
stock
underbarrel