Drop content_sao.{cpp,h}

Move LuaEntitySAO to a new dedicated file
Drop TestSAO (useless object)
Drop the old static startup initialized SAO factory, which was pretty useless.
This factory was using a std::map for 2 elements, now just use a simple condition owned by ServerEnvironment, which will be lightweight, that will also drop a one time useful test on each LuaEntitySAO creation. This should reduce server load on massive SAO creation

src/serverenvironment.h

@@ -468,4 +468,6 @@ private:
 	IntervalLimiter m_particle_management_interval;
 	std::unordered_map<u32, float> m_particle_spawners;
 	std::unordered_map<u32, u16> m_particle_spawner_attachments;
+
+	ServerActiveObject* createSAO(ActiveObjectType type, v3f pos, const std::string &data);
 };