Store all mucsub notifications not only message notifications

Previously we stored only message/subject change notifications, but if user
request also change notificaitons for affiliation/config/subscribers then
i don't see reason why we shouldn't store it as well.

src/mod_mam.erl

@@ -740,7 +740,7 @@ should_archive(#message{body = Body, subject = Subject,
 				#message{} = Msg ->
 				    should_archive(Msg, LServer);
 				_ ->
-				    false
+				    misc:is_mucsub_message(Pkt)
 			    end
 		    end
 	    end