Optimize appliance templates update from GitHub repository by only downloading when the repository

has been updated. Ref https://github.com/GNS3/gns3-gui/issues/2490

gns3server/handlers/api/controller/appliance_handler.py

@@ -39,7 +39,7 @@ class ApplianceHandler:
         controller = Controller.instance()
         if request.query.get("update", "no") == "yes":
             yield from controller.download_appliance_templates()
-            controller.load_appliance_templates()
+        controller.load_appliance_templates()
         response.json([c for c in controller.appliance_templates.values()])
 
     @Route.get(