Small changes to Gamma, moved BossFightFortress

Added GameModeEndReason to Gamma_OnGameModeEnd and Gamma_OnGameModeEnded
Added BehaviourReleaseReason to Gamma_OnBehaviourReleasingClient and Gamma_OnBehaviourReleasedClient
Moved BossFightFortress to it's own repo:
https://github.com/CookiesIO/BossFightFortress/