Vitality changes scale health, Purple Aura monsters heal others

- Whem monsters level-up, their health will scale from their previous value so
  that they have the same percentage of health as they did before they leveled
  up. That means that leveling will not particularly "heal" them anymore, but
  rather just scale their existing health up.
- Purple Auras now heal their teammates for 10% of their maximum health every
  time they themselves regenerate health. Added an effect for it, too.