Scripts/Gurtogg:
* fixed bloodboil targeting
* fel rage should only hit players

Closes #12193
Core/Creature: load correct vehicleid of difficulty mode npcs if set otherwise inherit from normal mode entry

Closes #8783
Core/Auras: Add a new stack rule to handle cases where the aura that is 'stronger' prevails (SPELL_GROUP_STACK_RULE_EXCLUSIVE_HIGHEST)