Download custom appliance symbols from GitHub

Fix symbol cache issue. Ref https://github.com/GNS3/gns3-gui/issues/2671
Fix temporary directory for symbols was not deleted
Fix temporary appliance file was not deleted

gns3server/handlers/api/controller/template_handler.py

@@ -51,6 +51,8 @@ class TemplateHandler:
 
         controller = Controller.instance()
         template = controller.template_manager.add_template(request.json)
+        # Reset the symbol list
+        controller.symbols.list()
         response.set_status(201)
         response.json(template)