Fixed scout's heavy mass parameters so he is now easier to pick up with the physgun

Fix Engineer with deformed bip_spine_2 physics collider, which caused some instability
Add some placeholders for "prp" less mercenaries:

• Scout without his dog tags
  
• Pyro without his backpack
  
• Engineer without his belt items
  
• Medic without his coat
  
• Sniper without his pouch

Fixes:

• Add a physics collider to Medic's left hand, with restricted constraints to mimic default aesthetic
  
• Restrict Soldier collar joint constraints to preserve default aesthetic

Initial implementation:

• Physics colliders for all spine bones
  
• Collarbone colliders
  
• Neck colliders