validate proxy.extra_routes

- add trailing slash if missing, and warn
- raise if leading slash is wrong (must not be present with host routing, must be present otherwise)

examples/server-api/jupyterhub_config.py

@@ -2,6 +2,8 @@
 # 1. start/stop servers, and
 # 2. access the server API
 
+c = get_config()  # noqa
+
 c.JupyterHub.load_roles = [
     {
         "name": "launcher",