Fix external-oauth example jupyterhub_config.py

- Roles need to be explicitly granted, otherwise you get a
  403. This example predates roles.
- Explicitly set bind_url - without this, JupyterHub itself doesn't
  seem to bind anywhere, and so you just get a 404 when you visit
  whatever port configurable-http-proxy lands on. This is probably
  a separate bug to be investigated, but in the meantime copying
  this from testing/jupyterhub_config.py makes this example actually
  work
- Set DummyAuthenticator as the default, so users can get started
  with this example

examples/external-oauth/jupyterhub_config.py

@@ -1,5 +1,16 @@
 import os
 
+# Allow anyone to authenticate
+from jupyterhub.auth import DummyAuthenticator
+
+c.JupyterHub.authenticator_class = DummyAuthenticator
+
+# Optionally set a global password that all users must use
+# c.DummyAuthenticator.password = "your_password"
+
+# only listen on localhost for testing.
+c.JupyterHub.bind_url = 'http://127.0.0.1:8000'
+
 # get the oauth client's API token.
 # this could come from anywhere
 api_token = os.getenv("JUPYTERHUB_API_TOKEN")
@@ -9,7 +20,6 @@ if not api_token:
     )
 
 # tell JupyterHub to register the service as an external oauth client
-
 c.JupyterHub.services = [
     {
         'name': 'external-oauth',
@@ -18,3 +28,13 @@ c.JupyterHub.services = [
         'oauth_redirect_uri': 'http://127.0.0.1:5555/oauth_callback',
     }
 ]
+
+# Grant all JupyterHub users ability to access services
+c.JupyterHub.load_roles = [
+    {
+        'name': 'user',
+        'description': 'Allow all users to access all services',
+        'scopes': ['access:services', 'self'],
+    }
+
+]