M16 Sling Mount fix

This will replace the default M16 with a more realistic one. The flashlight has been moved to the right side of the hand-guard, and the sling mount where it used to be now swings when the players move (sadly, it's not affected by gravity).

See it in action here: //youtu.be/eg0uUrlz_Zk

Special thanks goes to Rick Hoffmaster/SgtR007's tutorial on how to insert Jigglebones. You can watch it yourself here:

//youtu.be/JP9ce_ygeSA

Also, before anyone comments on it, the magazine clips through the front of the worldview magwell when Louis or Francis hold the rifle. This is true even in vanilla L4D, something I'm surprised I never noticed before.