Core/Spells: implement DoT clipping

* Core/Spells: Implementation of DoT clipping.
* Core/Utilities: Backport "EnumFlags v3.0" from master.
* Core/Spells: Switch AURA_REMOVE_MODE to a flag set.

src/server/game/Entities/DynamicObject/DynamicObject.cpp

@@ -252,7 +252,7 @@ void DynamicObject::RemoveAura()
     _removedAura = _aura;
     _aura = nullptr;
     if (!_removedAura->IsRemoved())
-        _removedAura->_Remove(AURA_REMOVE_BY_DEFAULT);
+        _removedAura->_Remove(AuraRemoveFlags::ByDefault);
 }
 
 void DynamicObject::SetCasterViewpoint()