API: pass 'server_state' through to admin clients only

- This will allow, for example, cull_idle_servers to be more
  intelligent when culling servers.
- This is only given to admin API users, because we don't know if all
  spawners expect their state to be made available to users.

jupyterhub/tests/test_api.py

@@ -207,6 +207,9 @@ def normalize_user(user):
             for key in ('started', 'last_activity'):
                 server[key] = normalize_timestamp(server[key])
             server['progress_url'] = re.sub(r'.*/hub/api', 'PREFIX/hub/api', server['progress_url'])
+            if (isinstance(server['state'], dict)
+                and isinstance(server['state'].get('pid', None), int)):
+                server['state']['pid'] = 0
     return user
 
 def fill_user(model):