System tiddler syncing (#4987)

* First pass at a fix for system tiddler sync issues

* Add new filter syncFromServer

* Undo previous attempt at a fix

* Added a flag to control sync of system tiddlers from server

core/modules/server/routes/get-tiddlers-json.js

@@ -28,6 +28,9 @@ exports.handler = function(request,response,state) {
 			return;
 		}
 	}
+	if(state.wiki.getTiddlerText("$:/config/SyncSystemTiddlersFromServer") === "no") {
+		filter += "+[!is[system]]";
+	}
 	var excludeFields = (state.queryParameters.exclude || "text").split(","),
 		titles = state.wiki.filterTiddlers(filter);
 	response.writeHead(200, {"Content-Type": "application/json"});