Restore config dir handling

jupyterlab/labextensions.py

@@ -27,6 +27,9 @@ class BaseExtensionApp(JupyterApp):
     version = __version__
     flags = flags
 
+    lab_config_dir = Unicode(ENV_CONFIG_PATH[0], config=True,
+         help="The lab configuration directory")
+
     should_build = Bool(True, config=True,
         help="Whether to build the app after the action")