UMcS Mobsprites Fix

*Adds a way for UMcS-unique items to have mobsprites without having to disrupt the /tg/ files. Simply add "urist_only = 1" to the definition of any item you add, and place the sprite in the corresponding file in icons/uristmob/
*Adds two welder items, sprites provided by ShoesandHats
*Moves policetape.dm into modules/urist/
*Moves the policetape hand sprites into the corresponding icons/uristmob/ file.