Adds the gravity generator (#3183)

Works like it did in old code with some minor additions. The sprite is the one TG uses.

code/modules/admin/secrets/random_events/gravity.dm

@@ -16,8 +16,9 @@
 		return
 
 	gravity_is_on = !gravity_is_on
-	for(var/area/A in all_areas)
-		A.gravitychange(gravity_is_on,A)
+	for(var/A in SSmachinery.gravity_generators)
+		var/obj/machinery/gravity_generator/main/B = A
+		B.eventshutofftoggle()
 
 	feedback_inc("admin_secrets_fun_used",1)
 	feedback_add_details("admin_secrets_fun_used","Grav")